That black goo is a build-up of dead skin cells, sweat, skin oil, and hair. Same thing happens with my Omega Seamster, and every other SS link band I've ever owned.
I would not use a sonic jewelry cleaner because you don't know how the sonic vibrations are going to affect the electronics of the Apple Watch. Instead, make a soap solution from a few drops of dish soap diluted in a coffee cup of water, dip the bracelet in it, and scrub the bracelet with a soft toothbrush. Rinse under water, and repeat until the black goo is gone.