Instagram doesn't send a notification when someone blocks you, but there are several signs that can indicate you've been blocked by someone:
1. **You Can't Find Their Profile**: When you search for the person who may have blocked you, their profile doesn't appear in the search results.
2. **You Can't See Their Posts or Stories**: If you were previously following the person, their posts and stories will no longer appear in your feed.
3. **No Message Thread**: If you had a message thread with the person, it may disappear from your inbox.
4. **No Activity on Their Profile**: You can't see their likes, comments, or other activity on Instagram.
5. **Profile Picture and Posts Appear Unavailable**: When you visit their profile, you may see a default profile picture instead of their actual photo, and their posts may appear to be unavailable.
6. **Direct Attempts to Interact Fail**: If you try to tag them in a post, mention them in a comment, or send them a direct message, your actions may not go through.
It's important to note that these signs may also occur for reasons other than being blocked, such as the person deactivating their account or changing their privacy settings. If you suspect you've been blocked, you may want to ask a mutual friend to check if they can still see the person's profile and posts.