I would like to add to MeEtc's post that this subject is not to ive help with. It is not like "safa" is having a problem detecting who is blocking him, or who is appearing offline.
Block checkers do NOT work, as MeEtc and a whole lot stated several times before, however, there is a possibility to detect appearing offline contacts, but it isn't that much efficient. This "feature" is available in MessengerDiscovery 2.0.