Before treating the stain, apply an ice pack or cold compress to the affected area for a few minutes to help constrict blood vessels and reduce bleeding. This will not only prevent the stain from setting but also make it easier to remove later. Next, blot the stain gently with a clean cloth or paper towel to remove any excess blood. Then, mix 1 tablespoon of white vinegar with 2 tablespoons of cold water and apply the solution to the stain. Gently agitate the solution with your fingers, then let it sit for about an hour before washing the garment as usual.
For tougher stains, you can also try using a solution of equal parts hydrogen peroxide and dish soap. Apply the solution directly to the stain, let it sit for 10-15 minutes, and then rinse with cold water before washing.
It's worth noting that the sooner you treat the stain, the better chance you have of removing it completely. So, if possible, try to get the stain out as soon as possible to prevent it from setting.