From my understanding, you do not need a permit for Blackfin. But where there are blackfin, therecould possibly be a YFT mixed in. And it would suck to have to throw it back because you don't have the cheap permit. Plenty of discussion on this topic. Just browse and searchthe Bluewater sections of the forum. Plenty of info on the old forum as well. It is still searchable.
I am not sure how far out you are willing to go but the BFT are thick at the rigs right now. We took a trip to the Petronas (about 70 miles out)this past Saturday and the BFT were so thick they were hittingour jigs on the way down. We had 5 in the boxes within15 minutes. As far as the permits go you need a permit for your boat not individuals and it will run you $28.00. The permit is good for 1 year but they expire 31 December so if you buy one now you will have to renew January 1. The permit didn't specify what type of Tuna it just said Tuna. Here is a link with info and you can also purchase and print the permit there as well. http://www.nmfspermits.com/default.asp