I like all the Tillerman books, and the one's linked to the Tillermans, but I think this is the best of them all.
Jeff has a lot to deal with when his mother leaves him as a young boy, because she feels called to help all the other unhappy children in the world!! The story follows Jeff as he works things out - his relationships with his mother and father, his school life, his interests - and ends up balanced and with healthy perspectives. There's a lot of outdoorness and descriptions of nature in the book - the solitary blue of the title is a blue heron - and this adds to the general attractiveness of this story. Also, there are wise grown ups in it, such as Jeff's father and his father's friend, a priest. These grownups have their own troubles - who can escape that - but they provide solid support for Jeff.
This review doesn't in the least capture what a wonderful book this is, but I strongly recommend it, especially for teenagers. It's a really good companion for the teenage years.