Dying a black dash?

Yes you can. It is not hard and starting with a darker color that you are dying is not a problem either. Dash pads for my 57 T-Bird are only availabe in blue and my interior is white. I used the correct dye and it is still looking brand new 8 years later. One trick I like to do is to wipe the entire part down with rag fairly wet with laquer thinner right before I apply the first coat of dye. This slightly melts the vinyl and opens its pores to make for a very good bond between it and the dye. Be careful and don't wipe hard.....you just want to soften the vinyl slightly and not dissolve it. Ever since I have been using this technique my vinly dye jobs have had superior results and longevity.
 
You should get the paint from a mustang supplier if you want a good match. As for the wiping down the dash you might consider this spray for vinyl and plastic that I purchased at walmart or autozone, It is a spray that you use before painting and has a picture of a bulldog on it.