Pen² > si les dents sont bleues, je dirais que l'hygiène dentaire laisse à désirer, au contraire :

https://en.wikipedia.org/wiki/Harald_Bluetooth :
The traditional explanation is that Harald must have had a conspicuous bad tooth that appeared "blue" (i.e. "black", as blár "blue" meant "blue-black", or "dark-coloured").

(...)

The Bluetooth wireless specification design was named after the king in 1997, and the Bluetooth logo consists of a Younger futhark bindrune for his initials, H (ᚼ) and B (ᛒ).