If you have received messages about "WhatsApp Trendy Blue" or "WhatsApp Blue," please DO NOT follow the instructions that came with them. The messages were NOT sent by WhatsApp and there is no WhatsApp Trendy Blue. The messages were instead sent by cyber-criminals, who are attempting to trick WhatsApp users into visiting the fake WhatsApp website: "www.whatsappblue.com," where their cellphone numbers will be harvested and sold to rogue marketing companies or scammers.
On the fake website, WhatsApp users will be asked to submit their telephone numbers and invite their friends to share the same scam, by claiming that by doing so, they will get the new “WhatsApp Trendy Blue” feature. Currently, there is no “WhatsApp Trendy Blue.”
The Fake WhatsApp Website: "www.whatsappblue.com"

How the Scam Works
The scam will take WhatsApp users to the following fake WhatsApp website: “whatsappblue.com”, which will ask them to enter their telephone numbers and share the same scam with their friends (see image below), which will only help spread it to thousands of WhatsApp users.

This scam is similar to the following: