Looks to me like "black-stemmed" hydrangea 'Zebra'.

Marketed worldwide under the Black Steel series...

If not that actual breed, you may've just found a naturally occurring mutation.

Now, the reddish color does seem to occur up toward the tops of black stemmed hydrangeas. For example, my black-stemmed 'Zorro' lacecap's new stems have a reddish hue to them.

could be... the older wood is very thin but very woody and brown on this particular plant so i will have to see when it grows out if it is black stemmed. i would be happy with black or red! I really liked the white flowers and floret shape and floret size on this one so yay for my grocery store hydrangea find!! :0) it is the first plant purchase for my new white garden.

springwood gardens idk if you are still watching but the plant grew out with nice Black stems as you predicted. I think your ID was right on. So I believe this hydrangea to be hydrangea 'Zebra' as you suggested. THANKS