Harald I of Denmark had blue teeth. Perhaps he had black teeth, nobody is quite sure as the meaning of blau has changed over the years. His other great achievement was to unite the warring provinces of Denmark and Norway under a single king (himself).
Jim Kardach developed a system in 1996 that would allow mobile telephones to communicate with computers thus uniting two independent areas of technology. Whilst working on the project he read The Longships by Frans Gunnar Bengtsson (which I read when I was twelve and remember to be rather good). The novel is set in the time of Harald Bluetooth. Jim Kardach felt he was uniting warring provinces of technology so he named the project Bluetooth. It was never meant to be the actual name on the marketing: blue teeth are not a pleasant image. But the name that was meant to be used - Pan - was already taken. So bluetooth it was and bluetooth it ever shall be.
You can read a full account of the naming process here.
I was reminded of all this indirectly by a friend who mentioned how much he liked the old-fashioned phrase hat-tip which is used by bloggers to give credit to a source. He was reminded of it by a hat-tip that I put up the other day and e-mailed me to say that "In my mind, the tipped hat is always a trilby."
It set me wondering when hats started to be tipped in the blogosphere. I can't work it out with any accuracy but here's a graph that I managed with Google on the use of the phrase in the news over the last ten years.
The last month without a single hat-tip appears to have been February 2004, so I imagine that the phrase must have come into common usage some time over the next few months.
There's something lovely about the antiquity of these phrases, rather like the old-fashioned freelance to describe somebody who will work for anyone.
An original marketing slide for Bluetooth technology incorporating a (slightly altered) runestone image of Harald Bluetooth.