If you are using dichroic that is on CLEAR glass, you would add it last... as the Raku (aka Chalcedoy or R108 or whatever it's called depending on the maker) is an opaque color. If you put the dichro on the bottom, you would loose it with Raku on top. Could depend on what you are going going for!
If you are referring to dichro coated black glass, that is a different application all together, as you have to watch not to burn your exposed dichro. Covering it up too much may not be what you are striving for.
The black base glass also helps to bring out contrasts, in in a more dramatic way, for the sparkles of the dichro, or "Raku".