Since CLB is officially a on 12-level scale (according to Wikipedia), we can make some inferences.
Listening:
CLB9 = IELTS 8.0, thus IELTS 8.5 is almost surely CLB10, because 8->9->9->9 does not make a lot of sense.
Reading, writing and speaking:
CLB9 = IELTS 7.0, thus IELTS 8.0 should be the safe boundary for CLB10, because 7->8.5->9->9 does not make a lot of sense.
The battle will be between IELTS 7.5 and 8.0, so if 8.0 or above for R/W/S it should be safely CLB10.