Just found out a way to know if you have been blocked.
Open your browser, go to the marketplace, sign in (very important). Now, chose any item and click on "Add to cart as gift" then enter the recipient's first and last SL name and click on "find resident". If you receive a message saying that the person cannot be found in directory, it probably means that she/he has blocked you.
Now, sign out and do the same steps, [...] click "find resident". If you can find a resident while you are signed out but cannot find it when signed in, you know you have been blocked.
This is the message you'll get if you have been blocked : Could not find [First Last SL name] in the Resident records. Make sure you are using the Resident's full SL name and not the display name. For help on finding the Second Life name
Hope it helps,