WhatsApp has added a tiny, but handy new feature into their messaging app that allows one to know when the other person has actually read a message, or simply received it. A single tick indicates that your message has been successfully sent, but not yet delivered to the recipient. A double tick indicates that the recipient's mobile phone has successfully received your message.
The new blue colored double tick indicates that the other person has opened WhatsApp and read your message. Earlier, you couldn't say whether a person has read the message, or simply received it, with the double tick notification. The addition of blue colored ticks brushes this confusion away for good.