Lou,

I tested on Charlie's cycle at 11dpo (6dpt) and got a -ve, so I would definitely say with a blast transfer, the later you test the better! I got my first +ve the day after my 13dpo BT, so basically the day my AF was due, and it was faint even then!

Remember blast transfers historically present with lower HCG levels than day 2 transfer (or even natural conceived pgs) so I would say don't test until Sunday at the earliest, and even then it may be too early!