I'm pretty sure that the black is a sign of water getting in to the wood. The gasket isn't the problem in itself, bt may contribute to it by holding water next to an unsealed edge. Not sure what can be done with what is already there, but before you do any more sealing, you should probably check with Dave Grimmer at Pygmy.

I tried very hard to get all my potentially exposed edges 100% sealed on my Coho, and have not had any problems