I agree... Maybe try developing longer. The 3000b film has very deep blacks...I mean it is a silver diffusion contact print....not sure how the blacks could be blacker. Try pulling an unexposed shot to see the full frame in pure black.
As for instax, it is worlds sharper with truer colors than sx-70. Also it holds up amazingly well in storage. I have instax film expired 7 years ago and the colors are still true when I shoot with it. Really impressive.