@patiodog,
The term blue-gum is pretty common here, has been since I can remember....I think it is mostly used in the central and lower Mississippi river valley area....from Commerce MO, to the gulf....there is a high concentration of black folk in that area....I live toward the upper end of the valley.
It means a person that has a somewhat bluish {actually purplish} hue to their gum line.....usually found in individuals with very dark skin.
It is a real physical feature, and that's all it is....a physical feature.
As far as the "lazy" part....I've never heard the term used in a "nice" way, and if someone were to use the term, you can bet that ...lazy, useless, stupid...etc...is also implied. Bluntly, it simply means a dark nigger.