What kind of hydrangea is it? Depending on the kind it can rebloom. I have planted mine in full bloom and they do fine, I did not cut mine back either. I would wait until the frosts have passed before planting outside otherwise it will kill the present and future blooms for this year. When planting apply some acid supplement and fertilize in the summer.

The reblooming macs are generally not those offered as gift plants (greenhouse or florist's hydrangeas) out of season. No pruning or cutting back necessary or suggested other than removing the spent flowers back to the next set of buds. It is highly unlikely to bloom again this season and since these types of hydrangeas are coddled/manipulated into bloom at an very early age, it might not even bloom next season. And you will want to offer winter protection - it will not be hardy in your zone without.

btw, only extended cold temperatures will kill off any remaining blossoms or seriously damage the plant - it takes more than just a light frost to do so. But since these types of hydrangeas come from a very protected and sheltered growing environment, I would not be hasty in planting it outside. Wait until both soil and air temperatures approach greenhouse conditions before planting.....like early summer.

Hydrangeas sold in bloom at Easter are florists or greenhouse hydrangeas. While the florists types will often establish and become worthwhile garden plants here, they often do not make the move into gardens well in colder zones. Wait until after last frost - these are not cold tolerant plants - and then gradually acclimate to the outdoors as you have been doing. You can remove spent flowers to tidy up the plant but there is no reason to cut all back.

It will definitely require winter protection to survive in your zone and hopefully rebloom next year - but treat it as an experiment and don't be disappointed if it doesn't establish well for you - if it fails it won't be your fault. Greenhouse grown, this plant has been on a regime of growth hormones, special lighting, fertilizers to force bloom in a particular month and often with no regard to long term plant health - more fertilizer at this point won't be helpful.