You know, I gave this card a pass the first time I saw it... but now that I think of it, this card has weird flavor implications. So, when you've been bitten by a vampire, you become a vampire and... become a black creature? That's it? No negative ramification? No super-powers? That's what all those vampire hunters are trying to stop? That's a bit... Twilight-ish...

(BTW, you should probably replace the first line with the colour indicator; and you can make the card frame and colour indicator black by selecting "Black" from the "Colour" dropdown.)

done

Still missing the colour indicator checkbox. I agree with jmg that this ought to do something else. Either "Draw a card at the beginning of the next turn's upkeep", or preferably, give the creature... something. The Sengir Vampire ability, maybe. (That'd work better if this was an Aura with Flash.)

there, it has flying.

Now it's a free Jump. Also, memory issues so this would be better as an Aura. If you really want it to keep it as an instant, a "memory hack" would be to exile the instant itself so players could use it as a reminder (as if it were an untouchable aura).

I would make it into a basically better Vampiric Link:

Mark of Sengir (B)

Enchantment — Aura

Enchant creature

Enchanted creature has lifelink and is a black Vampire in addition to its other colors and types.