For the blood hunter's profaned soul subclass at level 7 they learn two features. One of them is called Reveal Arcana where it states

"At 7th level, your dark patron grants you the rare use of a dangerous arcane spell based on your pact."

Though it gives no selection nor any additional information of what the arcane spell is or could be. But it appears on the character sheet page.

Additional bug: the lvl 15 feature also has the same problem.

They are extra spells. If You check your character's spell list, there will be 1 (at level 7, 2 at level 15) extra spell(s) specific to your patron.