can itself result in hyper pigmented patch called as ACANTHIOSIS NIGRICANS and this looks like a bruise.
Other common causes for bruise could be
Lack of VIT B12 ,VIT C,FOLIC ACID
,VIT K etc.
Bleeding disorders like haemophilia
Use drugs like aspirin,pain killers,local or systemic steroids.
You must discuss with your doctor and get the required investigations done to look for he cause of these bruises.