Finality counters work on any permanent, not only creatures. If a permanent with a finality counter on it would go to a graveyard from the battlefield, exile it instead.

If you sacrificed an Angel to pay the cost of Shilgengar's second ability, use the toughness of that Angel as it last existed on the battlefield to determine how many Blood tokens to create.

Finality counters aren't keyword counters, and a finality counter doesn't give any abilities to the permanent it's on. If that permanent loses its abilities and then would go to a graveyard, it will still be exiled instead.

Multiple finality counters on a single permanent are redundant.

Finality counters don't stop permanents from going to zones other than the graveyard from the battlefield. For example, if a permanent with a finality counter on it would be put into its owner's hand from the battlefield, it does so normally.

You can't sacrifice a Blood token to pay multiple costs.

If an effect refers to a Blood token, it means any artifact token with the subtype Blood, even if it has gained other subtypes.

Some triggered abilities trigger "whenever you sacrifice a Blood token." These abilities trigger regardless of why you sacrificed that Blood token.