A Blast can be rejected in several situations:
the webpage does not host the infringing material anymore (because the content of the webpage has been refreshed).
the webpage makes a use of your content in a way that may be considered Fair use.
the webpage for which you submitted the Blast clearly does not make any illegal use of your content.
NB: if you disagree with the rejection, you will soon have the possibility to appeal the decision. A Blasty representative will then review your request and resubmit the Blast.