When teeth erupt, they slowly move through the gum tissue. Usually this is no big deal although some babies are quite fussy when "cutting" new teeth. As children get older, usually around 6 years old, new teeth begin to erupt then as well.
Sometimes teeth, incisors in the front of the mouth or molars in the back, can end up turning the gums a blue or gray color. This is called an Eruption Hematoma. This is due to the gums becoming thin in front of the erupting tooth and some minor trauma causing a bruise in that area. Often it doesn't take much of a trauma to do this, just normal eating or biting on a toy. It does not hurt. Just like a bruise, it will tend to go away on it's own once the new tooth erupts. No treatment is needed. It is a good idea to get your dentist to look at it to rule out anything else.
Occasionally, the area will appear swollen. If you feel it, it seems to be squishy or a fluid filled area under the gum. This is called an "Eruption Cyst". Again, it usually resolves once the new tooth comes in. Rarely, the area gets pretty big or has been there longer than we would like. In those cases we can assist the eruption with surgical exposure of the new tooth. It's pretty easy to do if needed.