So here is the idea I had about something that would please the great Lord Tzeentch.

Overthrowing a Black Ship and converting all the potential psykers to Tzeentch.

Is this even possible for a group of heretiks?

What kind of enemies do we have to account for?

Would it net less then a million infamy?

It's possible, but given the security on the Black Ships, and the fact that they obviously don't broadcast their routes, you'd have to not only get together a ship, crew, and army to take the Black Ship, you'd also have to find one.

Heck of a win if you can pull it off, they're very nearly a pocket battleship/battlecruiser in their own right. Getting onboard and starting a large riot is the other option but it would probably be less assured of victory due to the nature of the vessel which takes no crap from the passengers, follow it up with a more conventional attack if you can disable enough defences or kill enough of its regular crew.

Doubt there would be a lot of psykers left, I can see an Inquisitor hitting the "Flush Holding Facilities into space" button on the bridge if it looked to be going against their favour

Also remember that the entire ship is infused with a palpable aura of psychic despair, making any kind of revolt that much harder to organize. Also, the crew is made up of hypno-conditioned sub-psychics and psychic nulls, so they'll have an edge in combat psychology over anyone trying to invade.

You probably want the help of a heretical inquisitor to begin with. It'd require a lot of planning too.