It can not be a yellow belly black snake if it has blue spots! That would be a yellow belly blue spotted black snake !
I Found A Yellow Belly Back Snake With Blue Spots Is It A Yellow Belly Black Snake Or Not?
Could you have actually seen a blue racer? They can have yellow bellies with kind of a bluish body? The description is also similar to a speckled racer. This type of snake has a yellow belly and blue spots. Check out some of these pictures and see if this might be a candidate: images.google.com