The nose contains many small blood vessels that bleed easily. Air moving through the nose can dry and irritate the membranes lining the inside of the nose, forming crusts. These crusts bleed when irritated by rubbing, picking, or blowing the nose.
Dry conditions can cause crust in the sinus wall to form and when disturbed by picking on it or just blowing too hard the crust will remove itself with blood. Best solution is Saline, nasal saline solution is sold over the counter in any Pharmacy. I've been a Pharmacy Tech for 14 years. You learn a lot working with great Pharmacist.