Hi all =)

Hope you are keeping safe out there.
I'm hoping someone with more experience than I can help with a job I have!

I have been given a wedding ring to repair. I cleaned with hot water/soap and a toothbrush then ran it in the ultrasonic prior to working on it.
It's hallmarked 375 gold and appears solid yellow gold with a twisted tri colour decorated band along the centre in red gold, white and yellow.
When I heated the ring to start soldering (there's a small crack in the rose gold needing repair)... some black gunk started seeping out from between the tri colour band and decorative ring edge.
I stopped heating further just in case it was something I haven't come across and put it in some pickle!
Is it possible this is just more of the 2o years of dirt coming out or should I be aware of something else it could be?
I was able to clean it off with some elbow grease but it took some shifting.

Thanks in advance...
I've tried to attach a picture but it keeps failing!