@ehBeth,
Thanks for the link ehBeth, but I find no answer to my question...
I can see the "last active stamp" of the contact in the FB chat on my PC, but I can't even see the contact in the Facebook Messenger App for Android.
That's why I'd like to know if he blocked me, or if it is a Facebook error. It is at least curious that, if an error, it happened just with this contact concretely.