When yellowing of the fan leaves starts depends a bit on when you switched them to bloom nutes. If you did so when they first started to show pre flowers, around week 4, then they will finish earlier, around week 8-9, so yellowing at week 7 would be normal. If you waited to make the switch until vertical growth stopped, around week 5-6, then it's a bit early. In that case I wouldn't expect them to start yellowing until around week 9.
Some pictures would be helpful so we can see which leaves are yellowing. If you only switched to bloom nutes around week 5-6, then it could be a pH issue. If the pH is off you could be experiencing some nutrient lockout.