I keep checking to see if someone answered this because I’m curious and did some googling (I know we can all google 😉 For GIA at least, I couldn’t find anything saying there is such a thing as a B-colored diamond. The only place I saw a mention of a B grade was on wikipedia for a historical grading system no longer used.
GIA does mention that colored diamonds go beyond the Z range but doesn’t mention “B” grade for color.
“Diamonds in the normal color range are colorless through light yellow and are described using the industry’s D-to-Z color-grading scale. Fancy color diamonds, on the other hand, are yellow and brown diamonds that exhibit color beyond the Z range, or diamonds that exhibit any other color face-up. These rare specimens come in every color of the spectrum, including, most importantly, blue, green, pink, and red.