What RAM, DSG and Canyon Diver said. Whatever the flavor diff betw YF and BkF tuna, if any, it is slight. I think most probs relate to poor preservation, handling. Fish that taste or even smell like "fish" have already begun to spoil. Because they thaw/heat more rapidly. smaller fish spoil first + most.
Canyon Diver: "Thermal load" yet. Nice term and explanation; that's waaay uptown. Sounds like chef school stuff. But, cooked meat tastes better after it has been allowed a few minutes off the grill (out of the skillet) for the heat energy to even out one way and the juices to redistribute the other. During that time, the meat continues to cook. I agree, if you cook tuna until it's ready, you are serving it overdone.
Snatched (f/k/a Snagged): The blood thing is an interesting puzzle. All the authorities say to bleed tuna; however, I've cooked and eaten the blood; and, it's mild flavored, like the meat. Go figure.
UR--Take heart; this is just the warmup season for tuna. The size and numbers come in October, November. You're just tuning up for the season.