I have 4 pink ones in containers.
Can I keep them in a dark garage until spring??
Yes, just water them every now and then. I water mine about twice in the winter.
Not sure you need to :-) I've grown lots of hydrangeas in containers and even held some nursery 1G plants over winter in their original containers without any difficulties. A zone 8 winter is usually nothing to be very concerned about with regards to hydrangeas. I'd just leave them outside in a somewhat protected location.....perhaps tucked under some larger shrub?