I like both but I find its harder to hit a flat ball with the black 7. I don't want my shots too spinny but thats probably due to my style.
They both have good comfort and feel (Black is a touch more crisp, blue's a bit more smooth) - But I string low in the mid to low 40's with midsize frames.
It comes down to a trade off between penetration and good spin (Blue) or super -almost stupid- amounts of spin (BlacK). I find blue to be better suited for me with player's racquets.
But a baseline pounder that likes to rip the ball with good power and comfort should look no further with black.