What Causes Blood Blister On Labia?

I have had 1 black "spot" on my labia since I had my first child. I have just delt with it. Some times it hurts if I scratch it. I now have another one and this one is very sore. Looks like a blood blister. an old one like you could squeeze it and a stringy coagulated blood might come out. I am 39 years old no hx real history. I weigh 150 pounds and am about 5'6.