Unsubscribed contacts are the ones who have opted out of your mail list by their choice. By doing so, they clearly specify that they no longer wish to receive any email communication from your end. 

A blocked contact is created by a Freshsurvey user. This option comes in handy when there are very sensitive accounts/ customers who should not be disturbed or bothered by your surveys. Or when a contact has specified that they don’t want to be contacted.