How can I get the LED on an iPhone 6 to blink constantly?
I'm not sure what search criteria you're using, but there are countless flashlight/torch apps that offer this function. Usually they refer to an option in their settings for a Strobe effect - this flashes the LED.
I've just done a search on the iOS App Store using the term &quot;flash light&quot; and the first three hits offered flashlight apps with a strobe option:

Flashlight by Intellectual Flame Co., Ltd.
Flashlight by MobileTrends Inc.
iTorch Flashlight by Pixelinlove Ltd

NOTE: I have not personally used any of these apps, nor am I affiliated in any way with the developers.