In version 0.0.7 of the Pimoroni UF2 they have taken out the backlight method for Explorer. It is still there on Display and essential! Are you using the latest version?
Looking at the screen with black background set (0,0,0) it is quite bright and the true black fame is much darker.
I can see in your photograph that you are getting the similar ‘burn’ marks on the background that I saw. It does not appear to have done any permanent damage. Perhaps you could just use (128,128,128) grey rather than (255,255,255) full, brilliant white for the characters and so reduce the contrast.
Have you got an SSD1306 128 x64 screen. I’ve got that working effectively and have put up a tutorial here:
SSD1306 With Raspberry Pi Pico : 6 Steps - Instructables
No backlight to worry about.
Please, let me now how you get on with Pico Display when it arrives.
Not backlight problems here but it is slower and far less refined.