Hi Anniebelle, usually the plants like hydrangia from the florists have had the roots treated with some chemicals (harmless to us or animals)to stunt the growth as these are really large shrubs when mature, now that it is growing new leaf, I would pot it into a larger pot and slowly harden it off outdoors so it gets used to the new air and temp, maybe bring it indoors at night for a couple of weeks, that way, if you see it start to suffer, you can just keep it indoors as before, If it is happy outside, then you can eventualy plant it in the garden if you want, you could also take some cuttings about September time from shoots that have not flowered and pot them into some peat with sand added and they root quite fast, about 6 to 8 weeks normaly, I would keep the cuttings indoors though till the next spring, pinch out the growing tips so they bush up a bit, they also like well manured or composted ground as they like to retain the moisture rather than dry soil. a bit of shade also but some sun part day.
The blue Hydrangias like an acid soil to stay blue and if you want pink use a more alcaline soil, adding potting soil made for Rhododendrons will alter the colour of blue and help make it darker blue as this soil has acidity added to it.
Hope this helps some.
Sometimes the florist hydrangeas won't do as well in the garden as the ones that you would buy at a nursery, it's worth trying because I don't know that they'll do well long-term indoors, but if it doesn't do too well for you don't get discouraged and give up on hydrangeas, try one of the ones from the outdoor section of the garden center next time.
Did the florist give you any idea which cultivar it is? Probably not. If he did you can look it up in PlantFiles.
It is probably a mophead/big leaf type. I've not seen any other type of hydrangea used by florists. I looked in Plant Files and found all of these to be cold hardy to at least Zone 6A. So, I don't see any reason why you can't plant it outside. I also don't see any reason why you can't keep it in a container if you want to. I understand that the container will cause you to lose a zone, but that would still make you a Zone 7A, which should be fine.
I've got an Endless Summer hyrangea blooming in a gallon pot on my front deck right now. It's not going to be left in the container, but as far as I know it could be.
I don't know about keeping it inside. I don't have house plants because my cats play with them. Maybe someone else can advise you on that.
This may be a case of divine ignorance or just simple beginner's luck, but I, too, received one of those potted hydrangeas a few months ago. I'm in zone 8b and had no idea about root stunting chemicals or hardening off, just took the thing, stuck it in the dirt in semi-shade and watered it like crazy. Blooms lasted a few months, chopped them off and the bush is growing by leaps and bounds; has already more than doubled in size. Stick yours in the dirt and be prepared to water it, they truly are water hogs.