Blender model adds extra materials when imported to Unity

Without files hard to say for sure, but my guess would be that this is because of blender's way of storing a set of polygon texture references separate from materials and in some cases also exporting that. This happens usually when you select texture in uv view while having faces selected; blender links the texture to only selected faces.

There's (at least) two ways to fix this:

• Blender way: select your object, while in edit mode hit A to select everything, go to uv view and select everything there as well. Then at bottom of the view select texture. (this way every face has the same texture and as result there should be only one material named after texture you selected)
• Unity way: I prefer this way and am pretty sure this would be the better for your situation as well: in unity, open model import settings, find second last item Material Naming and set it to From Model's Material. This way should make sure no other than explicitly created materials are imported.