WhatsApp is worth billions of dollars, and there seems to be a good reason for that: many of you are using it, at least according to the flood of tips we are now getting. Today on the backend for WhatsApp they have evidently thrown a switch to make the double checks blue for when someone reads your messages.
If you see two blue check marks next to your sent message, then the recipient has read your message. In a group chat or broadcast message, the check marks will turn blue when every participant has read your message.
You do not need to update the app, as this is something that the WhatsApp team has done server side. So just enjoy the blue checks.