There are some new bus services to CH Girls but I don't think they go from Blackwell/Barnt Green.
Check out http://www.thegreenbus.co.uk
for details of all the current school bus routes.
Unfortunately they do make some route changes from year to year depending of the pupil home locations for that year. You could also give them a call to find out new planned routes for 2009 - they are very helpful.
Probably the best KE school access for Blackwell is KE 5-Ways - the 881 (my son's bus) service goes past there and I think there is a bus to KE Handsworth from that area. However 5-ways is a co-ed school so you may not be interested.
Might be worthwhile exploring Sutton Coldfield Girls Grammar seeing as there is an excellent train service.