I use matte black inks (Epson 3880). I do rub it gently, but only after rinsing it first, sometimes with warm water. The ink comes off right in the first rinse of water under the sink. I just use my fingers to get the rest of the goop off. I always apply a prime layer, let it dry, and then do the transfer. My transfers are good, then when I rinse, often the blacks will fade, and other times I will get pin pricks so it leaves a pattern of tiny spots.

Your problem is with the matte black inks. I think it contains some greasy component that keeps it from be locked into the SuperSauce.  If you use the Luster Epson profile that will use photo black  ink.  That profile is richer in color than the photo gloss profiles. If you are using the classic film you can skip the de-slime step and apply a quick sealing coat of the SuperSauce Solution.  Use single strokes quickly and do not brush hard or you will dissolve the image. Just flow it on and let it dry.