YES! I finally got bloodthirsty with the damn crossbow, after previously getting four kills three times, once in team deathmatch with the tribolt attachment and twice in hardpoint with no attachments, I got it on hardpoint in Overflow with the tribolt on.
This was my complete setup if it helps anyone: MP7 w/ silencer, crossbow w/ tribolt, lightweight, scavenger, dexterity, and tactical mask, and 1 concussion grenade. I actually had to get several kills with the mp7 in between in order to refill the crossbow with scavenger. I had tried this before with guns I picked up, or the MP7 without the silencer, but usually I'd get quickly killed after by some random seeing me light up as a red dot on their map.
As far as strategy, try to get a kill with both of your first clips, impact kills if possible as they will give you scavenger packs while an explosive kill won't (an impact kill requires, I think, a direct hit with all the arrows or a hit on a damaged enemy). If not, you will have to use the mp7 or knife to get a few scavenger packs after your first two crossbow kills. If you are expecting to get into a gun fight with an enemy who can see you after the first two crossbow kills, switch to using your mp7 until you see an easier target (as a plus this will also allow you to refill your clip). Remember, even if you get a bloodthirsty medal it won't count until you get 5 kills overall with the crossbow. It took many tries and a fair amount of luck, but I finally got it!