(D) and (B) actually work together to change a black to a blue, and a chocolate to a lilac. Like the (B) series, there is only 2 genes in this series.
D = "Dense" Dominant over (d). Colors usually have brown eyes. All black and chocolate based colors are (D). Examples are black tort, chocolate tort, black, chocolate, ect.
d = "Dilute" Recessive over (D). Colors have blue-gray eyes. Changes a black or chocolate to a blue or lilac. A dilute color has the same color pigments as its dense counterpart, but the pigment granules are spaced unevenly, thus creating a less intense appearance. Examples are blue tort, lilac tort, blue, lilac, ect.
So here's a question. What color-based (lilac, chocolate, black or blue), would B? dd be?
Answer: B? dd is the dilute version of black, which is BLUE! That rabbit would be blue-based. Thus, a chocolate-based rabbit would be bb DD.
Now none of this came from my own brain. I would like to thank Ellyn Eddy of Rabbit Smarties (you can Google it), for her book, "It's Easy as ABC: About Bunny Colors", a book on rabbit coat color genetics and a practical guide on raising colors in your rabbitry and applying what you know in rabbit coat color genetics to get an accurate picture of all the colors you could produce with a pair of rabbits.
~What's with the question marks? I put question marks next to the letters to say that with that rabbit, we don't know what it carries. We don't know what its recessive gene is, or even if it has one at all. the recessive gene cannot carry anything, so that's why there's two dd (for example), and a question mark after the dominant gene, I'll talk about finding out what genes your rabbit carries and more on recessives in another post. This will be a long series.
~The reason why I say "blue-based", or "black-based" (ect), is because those aren't the genes for just a pure blue or black; well, they are, but they're also for each individual blue- or black-based (ect), color like black otter, blue otter, black tort, blue tort, orange, cream, Siamese sable, smoke pearl, chestnut, opal, squirrel, chinchilla, a lots, lots more.
Hopefully when I'm done with it I can incorporate all of this info in a single article on my website!