I don't think Frank has given the correct answer to a question yet. Even money on a Blackjack is insurance. When you strip away the BS (not basic strategy) you are betting that the dealer's hole card is a 10 with a payout of 2 to 1. This would be an even bet if 1/3 of the cards were 10's. Unfortunately, 4/13 of the cards are 10's, so a fair payout would be 9 to 4. Unless you are counting cards and know that the deck is rich in 10's, you should not take this bet, whether you have blackjack or not.
The fact that you have a bleck jack does not necessarily indicate that the deck is rich. The only information you derive from the fact that you have a blackjack that is of any value in making an insurance bet is that one 10 has been removed from the deck, making the insurance bet even poorer than usual -- more significantly so in single deck than in multi-deck. Of course, if you are like Frank, and your very presence at the table repeals the laws of probability and logic, you just have to do what works for you.