That's okay- we all miss it. There are a couple ways to tell if the snake is blue, even if you can't detect a visual change. First, the snake will become inactive. If you haven't seen the snake out for an evening or two- most likely he's blue. I have three snakes right next to my TV- it's so easy to see who comes out at night, and quite noticeable when they don't.
Then, if you get in the habit of weighing your snakes before feeding, you will notice a larger than expected weight gain, which means the snake hasn't pooped, which usually means the snake is inactive, which means the snake is blue.