First, let me state that I know/knew nothing about hydrangea bushes; so I asked my wife who loves them.
Soils acidity affects bloom colour as well as genetics; your blue hydrangea needs aluminum which is more available in acidic soils. I assume that is why 'miracid' is recommended.

It seems pruning method and timing will affect blooming. Check out this link:

http://www.hydrangeashydrangeas.com/pruning.html

Hope this helps; I learned something today. :)