Hello
It occurs due to blunt injury where capillary blood vessels breaks and internal bleeding occurs without breach if skin which clots gradually..
So first immediately after injury it looks red then blue and then black ..
Next Steps
No need to worry
Just apply thrombophob ointment which break ur clots (skin may look red again for some day ,its normal don't worry)
after 3 days apply mometasone cream there..
U will see significant results within 7 days👍