Finally, when the double tick icon, gets a black shading, it means that the recipient has read the message. The privacy settings within the Signal Messaging app allows you to disable read receipts, which means that people will not be able to see whether or not you have read their message, and on their end, the message marker will be stuck on the double tick with no shading.
You can download your copy of the Signal messaging app from the link below.
Signal for Android – Click Here.
Signal for iOS – Click Here.
