One of my PC's tried the Celestial bloodline with his character, and both of us consider it pretty weak. I think they would have worked better as "bloodline substitution levels", where all the abilities are granted at the same time each +1 to LA occurs.
The "magic levels" would be 3rd, 6th, and 12th (add 18th and 24th for particularly potent bloodlines). If the character doesn't take a substitution level at the proper time, the character is assumed to have manifested all his bloodline abilities and cannot "go back" and change later. Some bloodlines would stop at +1 LA, while others may go up to +4 or +5 LA. At any rate, the bloodline should be about as powerful as the LA suggests.
For example, a Celestial bloodline would start as a LA +1 race with Aasimar-like powers to people who gave up 3rd level to his bloodline. As he gives up levels at 6th, 12th, and finally 18th, he morphs into a Half-Celestial (which happens to be a LA +4 template).
The player doesn't even have to be aware of his heritage at character creation, the DM can simply bring up the notion when the PC's about to turn 3rd level and let him choose whether or not he "accepts his heritage". If he doesn't, the PC remains a normal member of the original race and the heritage is "pure fluff". If he accepts, his heritage gains a little "crunch", perhaps developing into a lot of "crunch" later.