I am hearing that they are responding to petitions to review, for which they provide a button to the blog author. If one of your images gets blocked, you click that button and it initiates review. The review is conducted by humans. So, that system is functioning, as far as I can tell. I am not sure if Tumblr actually tells you why a specific image got blocked.

"People spend time and effort on making a good upload." Well...{humorous observation follows:} 90% of Tumblr is not content created by the author of the specific Tumblr-blogpage! Tumblr consists of reblogging of reglogged reblogged reblooged content. Probably 50% of the 90% does not even credit the page they got the content, let alone the actual creator of it. It is the land of copyright nightmare.