I find there are numerous solutions to clean matte black, the real trouble is keeping it clean. My matte black frame stays clean for about half a ride. Just have to live with them being dirty looking maybe. I might try a good car wax soon and see how that pans out, idk.
After further research, I heard that soap and water is the best for black, keeping in mind how applied, due to the electronics. Wax and silicones of any kind should be avoided on Matte surfaces according to Specialized dealer, which is my non ebike.
For cleaning my ebike I use an atomizer to spray the water onto a diaper rag and wipe the bike. This is a for sure way to contain any liquid from getting in the wrong place in addition to potential shock issues, which I would hope would not arise regardless. I went with White on saturday to avoid the dilemma.
Cleaning a matt black frame is actually very easy. Normal Silicon spray kind of works, but the best product that I have found is 'Fork Juice' suspension lube
I use it on my suspension, but quickly discovered that it gives the cycle frame a superb smear free finish as well. http://www.juicelubes.co.uk