With flintlocks black 3F works the best for me. I prime with 3F as well and have no problems. My smoothbore trade rifle has a Chambers lock and after I got the flints set right the ignition is reliable to the point I don't think about it much. I shoot shot during Turkey season and use patched round balls in deer season. My gun is accurate to 50 yards with a PRB and about 25 yards with shot.
Synthetic black is rumored to be unreliable in ignition in a flint gun and is supposed to be more corrosive as well. I have no problem cleaning the gun it just takes a little more time than smokeless guns do.
