Ah, dear friends, how heavily the disillusionment that comes with knowledge weighs upon the soul.
A "blue peter" is not a sign of prolonged sexual frustration, nor is hoisting same a suggestion for a cure.
The Blue Peter is, in fact, a maritime flag used to signal to all ashore that the ship is preparing to leave port, and anyone who has business with those aboard should wrap things up quickly. The flag is blue with a white square in the center (originally six white balls) and should be flown at the head of the foremast--flown elsewhere, it simply stands for the letter "P". The odd name seems to come from the heraldic designation "blue pierced with white", or it may be derived from the French word for departure, partir.
Corroboration: www.fotw.ca/flags/host.html