Why has my black pearl turned blue?

by CRAIG
(Marseille, France)

I was given a black pearl for Christmas, and have been wearing it on an 18 carat gold chain around my neck for about a week. I have noticed that it has changed colour and is now becoming a really attractive blue colour. But, I am beginning to wonder if it is a real pearl or a fake because it has changed colour.

Is this a normal reaction for a pearl? Sorry, I know it sounds dumb to ask such a question, but I know nothing about pearls, and this is the first one I have ever had.