My two cents and alternate opinion-
Some people get busy and don't send out thank you notes quickly especially with a baby. I didn't get some out to my MIL once within a week when my kids were a baby and toddler and she sent me a scathing letter. I know I should have done it sooner, but it's not like I was sitting on the couch eating bonbons.
Kohls is not an expensive store and some people just like to have new stuff for their baby. Apparently the thank you is important so give her a call or email. I would donate the blanket, too.