Ok so removing power lines in Photoshop is trivial. However I am interested in how the process might work with analog photography, in the darkroom.
How would you remove power lines from a photo in the darkroom?
Not in the darkroom, but at the retouching (spotting) table.
If you're working with large format film, you can paint them out on the negative. But that's risky. Most of the time you retouch the print.
I never did any of it myself, but in college there were artists who would advertise their services in the photography department. I saw some of them in action.
They worked mainly with inks, bleach, and diluted developer solutions. Their tools were very fine tipped brushes and knives. And hands that would make a brain surgeon jealous.
My courses were journalism-based, so that was not allowed.
The big photography supply stores like B&H or Adorama still sell supplies for retouchers. So I guess there's still a market for old-school 'photoshopping'.