> I don't think they should change so suddenly?
>
Are you sure it was sudden? Could you just not have noticed?
Black gums are usually just a sign of deep black pigmentation which is highly desirable in many breeds.
Pale pink, grey or white gums are a sign of shock, anaemia, heart touble and several other conditions
If you are at all worried and/or there are any other 'symptoms' then contact your vet.