I use them on certain occasions. If I am in a night op I use black BB's. I use them in the dark because I won't be able to see or track my rounds anyway. I enjoy tracking the BB's in the daylight so I usually stick to white BB's, however once I got a tightbore, and used a hop up patch similar to the patches Spazz produces my BB's are a little more stable and predictable so I used black BB's to test it out the other day and had good results.
I have found that the white BB's are easy to see in low light situations both coming and going. The black BB's are less obvious and I feel the element of surprise and in some occasions location is kept a little better with the black BB's.
Black BB's are growing on me and I would be interested in testing out .3's as well