yup elytho, u are right, but I think hum wants to know if the person comes ol when he is not there... for tht only I referred Messenger Plus... IF u want to check whether u r blocked or not, then visit following URL
http://www.mess.be It has a block checker. When u enter the ID, then it tells whether the particular person is online or offline.... if he/she is online but not shown on your ID, then u r being blocked...