I'm awfully glad that you have another appointment coming up with the dentist.
Is this spot new to you, or could it have been there during the time you weren't paying any attention to your mouth?
I have a dark spot way back on my gum, and my dentist said that it was called a "tatoo." I don't know what caused it. It is near a tooth with a metal-based crown. So.....hold onto that thought through the weekend...that it could be something very simple. I suppose that it could also be a chewing injury, such as poking your gum with a sharp bit of food--a chip or popcorn. It could be a small broken blood vessel under the skin, which would look dark or blue.
Iknow that you aren't going to ignore it until you get an answer, but don't panic. It might be very simple.