Hi tullio, SKH provides an intriguing explanation for the use of the letter B for magnetic fields, however this does not explain why the letter H is also used for magnetic fields.
Therefore, I am sure that the letter B was not chosen in honour of Mr Biot, in fact, I believe it was used long before the Biot-Savart Law was described. I will add to this once I find the origin for B and H. I suspect they are the first letters for something magnetic in a foreign language.
Edit. Well, I suspect that the letter B was selected because the field lines always point towards the North Pole and the word for