Definitely some sludge, and I see a lot of gasket material on the pump face so I'd be extra thorough removing this (without damaging the gasket surface) but nothing unexpected. my bet would be the bottom part of this channel is the leak point. You'll definitely want to replace that gasket (I recommend felpro's)
P.S. if you don't own the tool to remove the harmonic balancer, The one you rent from advanced, oriley, and autozone ALL don't have the correct bolts you need (ask me how I know...). Autozone is the only local place I've found with the right bolts for rent or purchase. They're cheap, but they're a separate "GM bolt set". Hope this saves you an extra trip to the store!
Nice work so far!