You’ll be able to distinguish blood blisters from fluid-filled friction blisters, because the raised area will fill with blood as opposed to a clear liquid. When you have a blood blister, deeper layers of the skin are affected, and the cells above the blister die off.
The skin’s blood vessels sustain some mild damage, often dilating as part of an immune response to the dying cells, and inflammation occurs. Initially, the encapsulated blood is a light shade of red, but the hue will darken with time.
Depending on how you acquired the blood blister, you may experience pain at or around the site, and you may notice that inflammation is present. Additionally, blood blisters may be itchy.