I was having a similar problem, yellowing leaves and big black blotches, eventually in a do or die moment, I stripped all the offending looking, or likely looking leaves.
I now have a marvellous display of twigs with lots of blooms at the ends !
My fuchsia guru said "the bad weather".
Some were affected by rust as well, but that wasn't the sole cause. How the blooms are staying put in the wind and rain that we've got outside atm, I'll never know !
:P