Alright...curiosity got the best of me and I dug it out. Sorry the image isn't real good, but the family was asleep and I had to use the cel camera
It isn't really a tax stamp...but it is a "production" stamp. It reads: Factory No. 2153, 3rd. Dist, State of New York.
The blanket still looks great and I am pretty sure the sticker has been on there since back in the day. I have no idea why it hasn't caused discernible discoloration to the blanket, but it is minor at worst.
I have probably had this nearly 20 years if not more.