Yes, that is actually true. Different soils depending on PH levels and nutrients levels can minimally or drastically affect the color of hydrangeas. You can read an interesting article regarding this in Fine Gardening Magazine August 2003. You can visit their site at: http://www.taunton.com/finegardening/index.asp
Marion
No, it's an old wives tale. The chemicals would work but never in that form. You can a "blueing agent" from garden centres, as Pliltaz says, the only other way is to drastically change the ph of your soil, not something you relly want to get into
Hi Anna,
It can be done, but it's not that easy and it depends on whether you want to change from pink to blue or the other way round. This site will help.
http://www.hydrangeashydrangeas.com/index.html
Newt