blue hydrangea

Hi,
I have several pink hydrangeas and one that wont flower but my mum bought me a blue hydrangea last week, its gorgeous! anyway, I know it needs to stay in a big pot or it will turn pink but is there anything I can do to keep it blue?
Homebase sales guy sait feed it with miracle grow ericaceous feed...I know this will keep the leaves healthy but not sure about keeping flowers blue or turning a pink plant blue...the hydrangea on the box of feed was pink too which didnt give me much confidence!
I read online that putting pennies in the soil will help? any ideas please? the cheaper the solution the better!
Thanks

ok, thanks

when I was able to get Ferrous Sulphate from work (as a by product) I added that to keep them blue....
so I'm thinking that perhaps it is just the sulphate (Ferrous or Aluminium) that is the important bit ie it's correcting the acidity level of the soil   

Cool, Thanks
as it happens my mum gave me some scouring pads today, I can only assume it was a hint for me to clean the oven out. I think I would rather burythem in some soil with my hydrangeas!
Thanks