What do you want out of your tarp? Do you want to shave ounces, or gain coverage? Do you intend on using it 3-Seasons, or 4?
The Blackbirds are long hammocks, so don't err on the side of too small. Most all of the popular tarps will cover the Blackbirds, some just give more 'ooops' room. As others have said, we've got some awfully good tarp makers lurking around this site. It would be difficult to find a bad one among them.