Some sort of sealer may work long term. Main thing to do is thoroughly clean the hypalon wear patches first with TFR (traffic film remover) dilluted in hot water. Wear gloves as it's an irritant. There are other products on the market including Rib Revive. Rib cleaning kit for Rib and inflatable boats
You can use a green scourer pad to help.
Dry off and use 3M Vinyl and Hypalon protector. One thing to be aware of, and I've never used it myself, is apparently it can make the tubes slippery when wet. 3M Vinyl Hypalon conditioner and protector for rib's and inflatables
If the black marks persist - ask el-capitain if you can forget the dress whites for the net trip out!