What Causes Painful Blister Like Spot On Foot?

I have a blister like spot on the top of my foot that is pretty painful to touch. The pressure from my socks hurts. It sometimes itches. Dark red on the outside of circle and purple in the center. Yesterday there was a black dot in the center and today there is a little white dot in very center. Hard feeling all around it. Any ideas on what this is? I am a 36 year old male with no major health problems. I work construction and thought at first this was a blister, but has been here for a week and isn't like any blisters I've had before.Thank you for your time.