is tap water ok for blueberry plants

It depends where you live, Bea. Blueberries like acid soil conditions so, if you live in a 'hard' water area then they will not like your tap water. In that case you would need to collect rainwater which is naturally acidic.

how can i convert hard tap water so that i can water my blueberries?

A water softener or a deioniser should remove the lime from the hard water. I have no experience of using this water on plants but many years ago I worked in the hospital service with artificial kidney machines and we used these to improve the water to the patients machines. In hard water areas you can purchase units to fit under the kitchen tap to give improved quality drinking water.