"Unable to send - Message Blocking is active"

Is your account paid up to date?  Are you trying to send links to potentially questionable material?  Are you trying to send a message internationally where international use isn't enabled on your account?  Has the user blocked you?  It probably isn't any of those but there is a growing number of complaints across carriers that messages from iPhones are not making it to Android users.  This is an Apple issue related to iMessage.