I just wanted to add a bit about doing BLS and Purees at the same time.

From what it says in the BLS book, doing a mixture of the two is confusing for your baby. You're telling them, on one hand, that you trust them to feed themselves but then, on the other hand, sometimes YOU are the one to feed them. It can be detrimental to their BLS development to do a mixture. I know that lots of people do it this way but there's really no need.

I'm the worlds biggest control freak and this is one part of what we do with DD that I struggle with - however, seeing my 11 month old eating a bowl of yoghurt with a spoon HERSELF just proves to me that we're doing the right thing.

BLS is awesome fun too.

So in answer to your original question, I'd wait until at least 6 months.