That is burnt oil and fuel deposits on the back Side of the valve, umbrella seals??...see positive seals press onto the guide boss and control the oil better...but I'd say that the guides and or valve stems r probably shot cause the int is sucking oil.
FWIW Umbrella seals ride on the valve and let oil get sucked into the guide easier.
Next time u remove valves....1st run a file around the lock groove cause u just carved out the bronze sleave.