I dont know, it doesn't seem to be the case. I created another one with www.blog and that time tumblr kept saying "This subdomain’s CNAME is not pointing to Tumblr."
atleast with the one that was just http://blog.purplestripesclothing.com/ tumblr said "Your domain is pointing to Tumblr!"
And even still whenever I type in blog.purplestripesclothing.com into the address bar it automatically ads the "www". Every other DNS setting I change propagates in less than 30 minutes on average, if its still propagating I can't see why it would take so long? If thats the case Ill wait but you know I dont know that might not be the issue.