The bruising may be due to the injury which you have not noticed, and causing soft tissue injury
or bleeding inside and resulting in dark bruise
Keep the leg elevated.
Wear elastic bandage
at the site.
Do alternate hot and cold fermentation.
and anti-inflammatory drugs to reduce bruise.
Take rest as much as you can for proper healing.
If the darkness of the bruise increases or you feel any fever than immediately consult your doctor for that. Hope you got your answer.