The Australian Red Cross Blood Service explains that bruising occurs after donating blood due to bleeding under the skin. It particularly happens when bleeding continues after the needle is taken out of the arm until the small hole in the vein closes.
Bruising after blood donation also happens when the needle is inserted into the arm and damage to the other side of the vein occurs, says the Australian Red Cross Blood Service. This creates a tiny hole through which blood leaks. Bruising also occurs when one of the fragile blood vessels below the skin gets damaged, resulting in bleeding. Bruising usually does not become immediately apparent during the donation period.
Large bruises occasionally develop after donating blood, notes the Australian Red Cross Blood Service. Some bruises appear while at the donation site, while others appear one or two days later. Although they tend to look serious, these bruises are generally harmless and disappear over time. It takes up to two weeks for the bruise to go away completely. While donors cannot always prevent bruising, they can minimize the size of a bruise by applying pressure on the area of the skin where blood was taken until bleeding stops. It is also important to wear the bandage for four hours after donating blood.