In On Becoming Toddlerwise, blanket time is mentioned on pages 45 and 48. Page 45 says for toddlers between ages 14-18 months to do time on a blanket 2-3 times a week for 10-15 minutes at a time. The purpose of this is to teach parameter skills. Page 48 says for toddlers between ages 14-20 months to do it in the place of roomtime at first, starting with 5 minutes at a time each day. You then work up to 30 minutes or longer.
So what does that mean for you? I know several moms who start blanket time well before 14 months old. I know some who never use it at all. The age you start it would really depend on your child. You can put a completely immobile 8 month old on a blanket to play, but if he can't move anywhere it isn't true blanket time. That doesn't mean you can't start doing it--it can be useful to teach the habit before he has the option of moving, but he isn't exercising any sort of personal restraint by staying on the blanket when he couldn't move off of it in the first place.
Ultimately, here is my take on blanket time. Blanket time is not something well defined in the -wise books. I say you take blanket time and make it what you see your toddler in need of. Does your toddler need help learning boundaries? Does he lack some focusing skills? Do you have a need for blanket time in situations such as church? Use blanket time as needed in your home. Personally, I like the use of blanket time as a time to teach parameter skills. I consider it a a structured playtime situation. Again, use blanket time as you see fit for your child and your family's situation.