I don’t know where the SU6 promise came from, but evidence that they say they have recognised the issue exists and are working on it but in the meantime have issued all/many? of the bug fixes for SU6 that are now in beta testing already, and black avionics screens are not listed there tends to suggest that the fix will indeed be in SU7 at the earliest.
Not sure if the current beta for SU6 is fixed in stone or whether they will at last minute add updates to it. I guess you can’t rule out a hotfix either at some point, but you’d think they want to run black screen fix through beta testing too.