I think that's a nice response. It fits my rule of "assume positive intentions".
Casual "Merry Christmas" wishes are likely to be well meaning.
I was raised Unitarian (sort of ;-) and not Christian. We did celebrate "christmas" but not "Christmas" exactly. Still I am not offended, even as a UU pagan taoist-wannabe, when someone wishes me Merry Christmas. I try to just say something like "thank you, you too" or "Happy Holidays" or sometimes I say "Glad Yule!" But I like "blessed be". Works nicely.

:-) So Hey, Happy Holidays Jim. :-)