Welcome, Routemaster!
I would spray with a dilute solution of dish soap in water--say about 5 ml soap per liter of water. Be sure to spray thoroughly, since it only kills the bugs it touches, and any you miss will repopulate the plant in 3-7 days. If you see baby ladybugs--they look like little black alligators--delay spraying for a few days to see if they will keep the blackflies' numbers down.

or spray with just fresh water knocking the black fly of and killing them and save the ladybirds
if the plant is up to it that is.

Yes a spray of water usually works for me to. But if persistent I use soapy water.