If you have received a message in your Inbox but it appears blurred out, this is because you are currently registered under the Basic (free) plan on site and have reached your messaging limit with that user. In order to unlock the conversation and send a reply, you would need to purchase a subscription.
Under the Basic plan, it is free to post a listing and search for matches. However, you will have limited messaging capabilities. Once you send a message, a returned message from that user will be blurred out until you subscribe.