What Does A Small Mole Like Darkening On Tibia Indicate?

My husband has had several bone operations over the past 30 years. He is almost 50. After a recent break to his tibia, when seeing a specialist, the specialist mentioned that he had a small mole like darkening on his tibia. What is this? He asked if this had bothered him and said that he would re-check it in 6 months.