A lump or growth is hard to diagnose even with an office visit, over the internet it is even harder as we can not even see the growth. A lump may indicate cancer, but many such growths are harmless. Many lumps are not painful or bothersome. It may be a fatty tissue deposit called Lipomas or a wart or a hematoma, but to be positive your vet will need to test the lump to be sure.
Any lump found on your animal should be tested to determine if it is a cancerous or benign lump. Your vet will want to perform a fine-needle aspiration or other appropriate test. It is performed quickly and allows some of the cells of the lump to be evaluated by the veterinary pathologist. This test will allow the vet to determine the nature of the lump and take the necessary steps to remove it. Some vets will leave it alone if it is not serious. If it is an abscess, he may just drain it and prescribe antibiotics. Lumps that are solid feeling, feel attached and fast growing should be checked as soon as possible as these are the ones that are more likely to be serious.
Here are a few sites for additional information and pictures to allow you to get an idea based on the physical characteristics..
Picture of Lipoma
Picture of Hemangiosarcoma
Picture of a mast cell tumor
http://www.vetsurgerycentral.com/mct.htm (mast tumor site)
Information on Canine Oral Papilloma virus
Now an abscess can be red and when it ruptures will bleed and dried blood can look black. Additionally, the brown recluse spider can leave a nasty bite that does turn black and spread as tissue dies. I wouldn't wait any longer. I'd have it looked at and a needle biopsy done as soon as possible.
I hope you find this information helpful.