Do you think it's a good idea to start a blog of letters to a pedophile?

Doesn't really sound good. It depends on what you are hoping to achieve from it. It sounds strange to me, sorry. Perhaps a mature thing to journalise it; put it in some kind of perspective. Talk to relevant professionals, maybe even police; he may have been involved with other pedo's therefore you may be able to indirectly assist police in saving other children.