Bluetooth was named after a late tenth century king, Harald Gormsson, viking king of Denmark and Norway. He was nicknamed "Blatand" which translates into Bluetooth. It was believed he had a tooth and gum disease which caused the discoloration of his teeth.

Additional Answer:

Bluetooth® is named for Tenth Century Danish king Harald Blåtand, known in English as Harold Bluetooth Gromson, who is reputed to have been so fond of blueberries that his teeth were stained blue.

And some more info:

It was named this as the king was most known for his unification of two warring tribes from Denmark and Norway. So, just as King Bluetooth connected these two previously warring tribes so does the Bluetooth technology with two electronic devices

Blue tooth were due to the blue colour of his crown he used to wear frequently.