Its really not a black gene, it in the TOV gene. (Touch of Velvet) People have called it the black lethal factor for years because Black Velvet was known as Black and it wasn't a common practice to mix colors like we have in the last 20 years. So the lethal factor can include Black Velvet, Brown Velvet, Violet Velvet, ect. Basically anything that Contains Velvet or TOV in the name can not be bred to another one with the Velvet (TOV) gene. Or, if you don't know the color name of your chinchilla, it will have stripes on the front paws and will be darker on the back and face with the color slowly fading down the sides to the belly.
Now this gene is dominate so a chinchilla will either have it or not. It can't be carried unseen, so if you get a standard from a Black Velvet parent you don't have to worry about breeding it to a Black Velvet. The ONLY time it could be unseen is if you bred TOV to a White or an Ebony. The solid Ebony gene or the solid white gene could mask the Black Velvet gene if the baby received both. On those kits I would assume the lethal factor when breeding, just to be safe.