If you gently bite the inside of your lower lip, you will notice that the tissue, held between the teeth, is sort of bumpy. Each of those little bumps represents a mucous or saliva gland, and each of these glands has a tiny duct that empties the mucous produced by that gland inside the mouth on the surface of the mucosa.
If, due to a traumatic incident one or more of these ducts are severed, the mucous produced by the gland may not be able to reach the surface of the mucosa and it may produce a bluish blister called a mucocele.
If you are concerned, if they have suddenly appeared, or are causing pain/discomfort then just visit your GP.