It's really hard to say what it could be. If you don't recall hitting it on something, perhaps it is a bite from who knows what...spider maybe. There are some spiders whose bites generate what looks like a blister in the middle with the surrounding skin becoming inflammed. Your finger doesn't look that bad, at least from what I can tell on the picture. I would take an anti-inflammatory such as ibuprophen and keep an eye on it just to make sure it doesn't get worse. If it does, then perhaps a visit to your doctor or even one of those miniute clinics is in order. Take care.
It does look like it could be an insect bite because of the black dot. How long have you had it? If it doesn't go away soon I would consult a doctor.
Are you on any sort of medication? I know that some medications (like BC, etc) can cause weird things like that. I would talk to someone who knows your whole medical history to figure out what it is.