You are quite right; most moles are harmless, but it's important to keep a vigilant eye for abnormalities! So kudos to you for being aware, doing research, etc. According to this website http://www.nhs.uk/Tools/Pages/Mole-slideshow.aspx, the ABCDE of suspicious mole detection are the following:
- A: Asymmetry
- B: Border
- C: Color ---
A normal mole usually has one uniform colour, whereas melanomas are
often uneven in colour. As the picture shows, a melanoma can have two
or three shades of brown or black, or varying shades of red and pink.
The darkening of a mole is a sign that it is possibly becoming
- D: Diameter
- E: Elevation or Enlargement
**There's more information on each of the ABCDE (included on the website), but color seemed to be the most relevant and concerning to you.
I recently had the same question myself (as I've noticed a darker center in my mole that was spreading its diameter). I can certainly understand the worry.
I went to the dermatologist who actually just removed the mole as a skin biopsy so that it can be tested to see if it's positive for melanoma. I have not gotten the results back yet, but the procedure was painless and quick. I'm not sure that a dermatologist can tell just by appearance if it's positive for melanoma -- that's why they do the biopsy -- so I suggest that you see a dermatologist (you might need a referral from your family doctor).
Best of luck!!