We have had this ivy in the kitchen window for years (5/6) and pruned it
every now and again, as well as watering and feeding as required.

Now this year, a couple of months ago, we noticed something like very
small black flies or soot? on most of the leaves, and a build up of
black whatever it was on all the joints. I had a ago at spraying it with
some suitable? bug killer but it didn't help. So a friend advises us to
cut the ivy right back to get rid of all the black soot deposits.
After doing this, we washed out the pot it was stood in and let it grow
again. Now it is going the same way as before, in that it is covered in
this black stuff, like flies or something.

Any idea as to what it is and what if any is the cure?

Thanks