I have this BBEG is technically a warlock/ Wizard whose patron is an undead fiend. So a lot of his stuff deals with necromancy. I’m trying to figure out how to home brew eldritch blast in a way that reflects that or even make a new spell entirely preferably not a cantrip. If anyone has any ideas that could help that would be greatly appreciated 

Ok, so a few questions then:

1. What do you want this new eldritch blast to do?
2. Why do you want it to be a spell not a cantrip?
3. What level are the players (or what CR is the BBEG)?

One thought for a thematic and "oh no it does what?" spell for this would be one which fires 3 shots, and deals 1d10 necrotic damage. If all 3 hit the same target, then the target ages by a number of years equal to the highest roll on the 3 damage dice, and the BBEG heals this many HP. The visual of the spell would be portions of their souls being siphoned to the BBEG.

Are you running your BBE from a stat block or a character sheet? If it’s a stat block, why not just give them the ability you want and skip the spell? That’s the way the newer monsters are being written.

Got to agree with Weird Potato there, I have been homebrewing my monsters and it's so, so much easier to just have the ability with something like "3/day" beside it than "can cast this spell, has 3 slots with which to do so". Running a warlock BBEG with a bunch of spells and abilities and looking up the spells made me just stick to what was on the statblock to avoid slowing the game down!

A mistake new DMs tend to make is thinking their baddies have to play by PC rules. They don't, and they shouldn't. In fact, monsters are designed to be broken in comparison with playable characters for the sole reason of action economy. A monster needs to hit harder and tank harder than a PC can because they have to survive multiple characters attacking them for several rounds. Even a level 20 build is going to fall short against a fully equipped party. It'll be underwhelming to fight.

You describe your BBEG as a wizard/undead warlock, which sounds to me like you built him like a PC. I recommend finding a stat block for a higher CR creature and adjusting the abilities to suit your needs. You can reflavor any ability to tell the story you want to tell. 

The players are level 10 and the way I have the BBEG background is he was introduced to dark works. And has slowly been infiltrating his men which are regular people who have been taken over by demon souls (changelings really). The way he does it is the use of this spell.

Oh, no no he is a CR 15. I used ideas from a PC but he is fully an NPC home brew guy. I was trying my best to describe what he is and the best way to describe it is a wizard esque type of character. 

Wait, so his spell turns people into changelings? That's a far cry from Eldritch Blast!

I'd recommend looking at the Aboleth for the way it affects characters with ti's attack:

If the target is a creature, it must succeed on a DC 14 Constitution saving throw or become diseased. The disease has no effect for 1 minute and can be removed by any magic that cures disease. After 1 minute, the diseased creature’s skin becomes translucent and slimy, the creature can’t regain hit points unless it is underwater, and the disease can be removed only by heal or another disease-curing spell of 6th level or higher. When the creature is outside a body of water, it takes 6 (1d12) acid damage every 10 minutes unless moisture is applied to the skin before 10 minutes have passed.

So you can rework it somewhat that the warlock/wizard BBEG has a spell which requires a save or the target becomes Cursed, and if the curse isn't removed then they become a Changeling.