Three steps to know if you've been blacklisted
Have you been sending messages to a person on WhatsApp Messenger and don't get a reply? If this contact's "last seen status" hasn't updated in while and you get a single check mark, you could be in trouble. This is what happens when someone gets blocked on WhatsApp.
According to the developer of the popular free texting application, there's no other indicator to know when a contact is blocking you. This is done for privacy reasons, so there's no obvious way to tell if this is happening to you. There are some clues that can help you to confirm your suspicions. Let's break them down.
WhatsApp uses timestamps to show the recent activity of its users. This is convenient, for instance, when you are texting a friend and realise another person is "online" or has been "last seen" a couple of minutes ago. This way you can begin a message conversation knowing they're free to chat away, as they've been using the app recently and you won't really disturb them.
Blocked contacts don't get "last seen" updates
This timestamp is also a good way to see if a contact has blocked you or not, as blocked people never get this updates. Perhaps cross-checking this information with a common friend or seeing the person in question using WhatsApp on their phones can tell you what's really going on.
Profile picture doesn't change on the iPhone
WhatsApp privacy rules also disable profile pictures for blocked contacts. This is, the people you have blocked cannot see your latest avatar. On an iPhone, this is quite evident as the current version 2.8.6 is struggling not to show you this. When you tap on a contact you suspect has blocked you, tap on the small profile picture on the right top corner to make it bigger. If WhatsApp loads an older picture, different from the small one you see on the contact list, this could be another indicator you have been blocked.
There's no official confirmation this is the case, it might be a bug with the iPhone app. During my testing I've seen this issue happening for a couple of days, where the new picture never appears in full screen mode when you are blocked.
Single and double checkmarks
Every tick next to the messages you send is meant to show the delivery status of the text. Basically, one checkmark means the message has arrived to WhatsApp's server and a second tick that it's been delivered to the recipient.
As discussed in this post, a double checkmark doesn't guarantee the text has been read. They could read the notification and dismiss it without launching the app and you would still get one checkmark.
In fact, if your friend hasn't opened the app in a while, iOS imposes limits on background processes, meaning you'll see a single checkmark until this person launched the app again (I'm researching what the backgrounding limit it).
WhatsApp lists a number of good reasons why you might be getting a single checkmark next to the message. Perhaps your friend doesn't have internet access, is busy and hasn't checked the phone or doesn't have the pro version in platforms other than iOS.
Top image by Drriss