To send invisible ink messages on an iPhone, open the Messages app and start a conversation. Type your message or choose an image, then instead of tapping the send arrow, press and hold it. This will bring up the effects menu. Make sure you're in the "Bubble" tab and then select "Invisible Ink." Finally, tap the send arrow again to send your message with the invisible ink effect. The message will appear blurred until the recipient swipes over it to reveal the content. This feature works on iPhones running iOS 16 or later with iMessage.