Hi, you can check it directly on the XFX website:
https://www.xfxforce.com/gpu/amd-radeon-rx-7000
In short: The B9 is the "Black Edition", the U9 is the the regular Merc. The former boasts slightly higher core clocks by default - 2220 MHz / 2560 MHz vs. 2175 MHz / 2535 MHz. In reality there shouldn't be any actual performance differences, as you should be able to manually clock the standard Merc to Black Edition levels fairly easily. Of course if both cards are in stock and are being sold for the very same price, there is no reason not to get the Black Edition.
