In response to John Kiernan, he wrote

"Could the rinse in water after buffered AEC be insufficient?
    If some phosphate buffer carries over into the haemalum
    there could be precipitation of aluminium phosphate, with
    attachment of haematein to the Al ions."

... but then I would expect to see these precipitates already after 
haemalum, right? But I checked the slide under the microscope, after 
haemalaum and rinsing in cold running tap water, the slides are clean, 
no precipitates. 
Unfortunately I had no time to do further experiments to clarifiy this 
problems, but I keep you all informed. I really prefer the sky-blue 
coulour after lithiumcarbonate to the dark-blue-grayish type of 
counterstain if I only blue in running tap water. 

Have a nice weekend.

Heike Grabsch, Germany