Whenever there is an unread notification (e.g. chat messages), it will blink the message menu with a color chosen by the user.
Install it from the gnome extensions page: https://extensions.gnome.org/extension/258/
First try to take a look at the FAQ. If your issue/question is not solved there, feel free to open an issue in the repository ;)
This project is licensed under LGPL v2.1 (see COPYING for more info)
Feel free to fork the project and send me pull requests with new features, corrections and translations. I'll gladly merge them and release new versions ASAP.