t. white,
Hi...I am no expert on this subject, but have had a couple scares with a couple of transient episodes of hemipenal prolapses with my male panther. Luckily, the problem spontaneously corrected itself but I was in near panic. Anyway, the second episode lasted a little over 2 hours and I tried soaking him in a shallow bath of sugar water (high osmolarity of the water may help reduce the swelling and allow for spontaneous retraction)...don't know if this really did anything as it retracted later by itself...I did attempt to manually reduce it but it was too much tissue and would not budge...
If the tissue is purplish/reddish black then this is most likely a prolapsed hemipenis (could also be a prolapsed intestine I suppose...) and see below. While I must admit I do not know much about seminal plugs...I believe these are whitish/yellowish and "stringy" looking.
However, the fact that your chams problem started last night and that you mention seeing a "scab" is very worrisome. The scab sounds like the tissue may becoming necrosed/dying (blood supply can get cut-off) which can be devastating. I don't wish to scare you but this condition can/will lead to infection...sepsis (systemic infection)...and death unless surgical treatment is not performed. Therefore, I would wrap the prolapsed tissue with a wet water (or saline)/sucrose gauze and take him to the vet ASAP!
I would say if it is dark tissue then take it to the vet ASAP...and don't worry about posting pictures...