If your character had a loyal companion Blink Dog by their side, how would they utilize them to best effect?

I recommend cuddling and belly rubs, though scratches under the chin are also pretty good.

To be serious, though, the two best ways to use them are 1). Take advantage of Keen Hearing and Smell to track other creatures, and 2). Have the dog run up to an enemy, bite them, and blink away.

Blink dogs are also excellent for fetch.  They can cut their total travel time in half, letting you fit double the amount of Fetch into a game of fetch.

Reminder: blink dogs have an intelligence score of 10 and are every bit as sapient and self-aware as the average villager.  A blink dog is your friend and ally, not your pet.  They're 'best utilized' as allies rather than attack dogs; blink dogs make decent scouts with a passable-ish +5 to stealth and a blink to escape with, provided you have a means of communicating with them.  They can also take a turn standing sentry for your camp unaided, since they're intelligent enough to be watchful on their own and know what to do if enemies show up.  In combat?  They're best utilized by getting them out of the way, since AC13 and HP22 will disappear in a single hit outside of Tier 1 play.

The relatively high int is probably the most interesting feature of a Blinkdog, now that you mention it. There's every possibility that a Blinkdog traveling with the party might have higher INT than one or more character in the party. If you've got a Warlock with Beast Speech in your party, the Blink Dog essentially becomes a fully functional party member... albeit one that requires an interpreter.

I mean, to be fair?  Blink dogs list "Blink Dog" as a language they can speak, and their ability to understand Sylvan means they can totally wrap their heads around normal mortal kith languages.  Someone could learn Blink Dog, and in turn teach their buddy Common, so that the blink dog and its kith companion could Han and Chewbacca around the adventure.  Nobody else has a clue what all the barks and yips mean, but to the player they're perfectly clear and they just have normal conversations with their buddy.  If that's not a cool idea, you don't know what cool ideas are.

Yes, that is a great distinction Yurei and one I wonder about (regarding intelligence). I'm wondering if I should play the dog as a NPC or let him play it as another PC. He has already said he wants to learn Sylvan from the animal.

Now I have to figure out the motivations of this creature. The story is that his pack was killed in a displacer beast attack and the PC found him lying near death, was then ambushed by the same displacer beasts, fought and killed them, and then healed the animal. The dog is going through some grieving right now.

So you have an animal that is as smart as a villager, was found in the jungles of Chult (lived there it's whole life?), and has a completely different world view. The Han/Chewie analogy is a great analogy. I'll be honest I was thinking more of a companion at a dog level and wasn't paying attention to its high INT level. This player was feeling a bit down with his character and some of the things they've been through and I wanted to reward them but with something different rather than just a usual magic item/money way.

100%, it's an NPC. I think motivation is pretty well covered with the backstory... the Blink Dog lost its family and was near death, and now this person not only healed the Blink Dog but also defeated the creatures that killed its family. That's more than enough motivation for the Blink Dog to stay by their side... where else is it going to go, after all? Maybe it's hoping to start a new pack some day, but for now it's just happy to have a friend.

I think another thing to keep things interesting would be making sure to not just treat them as a human with four legs who barks, but rather a dog with human level intelligence. So it would still have dog-like instincts and attitudes... but it simply understands questions and commands easily and can fulfill more complex tasks. My mind goes pretty immediately to the dogs from Up.

So it's very reliable as long as no one mentions squirrels?

Considering the number of people who want to "tame!" the "cute!" goblins and kobolds (which isn't just stupid but uncomfortably close to colonization), I'm not surprised people think blink dogs are pets. That stuff gives me the creeps for some reason. Not treating monsters with respect is a fast track to death in my games. But the Han/Chewy analogy would be awesome!

Only problem with blink dogs is that although they understand Sylvan, they cannot speak. Now grab a druid with awaken and you can have a nice talking normal dog or bear, or anything else really. 

The PC is a ranger and has speak with animals. So seems like it'd be pretty easy for him to learn Sylvan from the Blink Dog.

Normally I use a sword or axe knock an enemy back and have the blink dog perform a bite move while staggered, I also have they sniff out gas traps and such. Or if the DM allows, blinks inside the enemy causing them to explode.

oh shit I played in an Out of The Abyss campaign with a new dm the changes he made to the module were nightmarishly bad but that's a story for another time;) anyway the paladin asked if he could have a blink dog companion and to my suprise he said yes later in the campaign when

 we uses the blink dog to teloport out leaving the NPCs to die these weren't the module's orignal NPCs they were diferent annoying charcters that i'm astonished we managed to get through 3 sessions with out them driving us mad before the underdark did not to mention the DMPC 

If you wanted to ensure that the Blink Dog could keep up with the rest of the party and not be one shot in every combat, you could give it a few levels as a Warrior using Tasha's sidekick rules. This allows it to gain more hp by giving the creature more hit die, allowing the creature to gain a higher proficiency bonus, be able to get ability score increases at certain and some features for being a warrior like skill proficiencies, Second Wind, ability to wear armor (which could be a cool side quest to try an obtain dog armor), and more.

You don't have to level the Blink Dog up every time the characters level up if you think it would make the creature too effective, but even a few levels give the creature much more survivability and a better chance to stay relevant in higher level combat and can be used as an reward for the player training and fighting alongside the Blink Dog in creative ways.

Edit: If your PC is a Beast Master Ranger though and the Blink Dog is his Animal Companion, then this wouldn't really be a good idea. Maybe you could incorporate some features from the Primal Beast Companion instead.

These are great ideas Rough! The characters are Lvl 6 right now, and I upped the Dog to a 'Pack Leader' and gave it more hit points so it could be used in battle with fear of immediate death. I'll checkout the sidekick rules for sure, as well as the other options.