Ellen, there are a number of possibilities. What I'm worried about is the source of the possible blood.
With her being overweight it might be difficult for her to groom her anal area and the area can look black and dirty.
However there are instances of intestinal bleeding that can be very serious.
There is no easy fix that I can share with you.
I really recommend that you take her to your veterinarian. He/she can do a complete physical. Be prepared for some blood work. It sounds like Sebastian has multiple issues: possible intestinal bleeding, possible lung problems, skin issues. All of these together with being overweight can be quite serious.
I know that you want the best for your Sebastian. So do I.
Please take Sebastian to his doctor.
I wise you and Sebastian the very best.
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Thank you and Happy Holidays!