blackfly on my dhalias

Not heard about the garlic but my dad used to spray his runner beans with a weak solution of washing up liquid in water - seemed to cure that problem for him. He started the minute he spied one!

Soak some garlic bulbs, liquidise them, strain and dilute with water in a spray. Slugs and black/ white fly don’t like it, but you won’t smell the garlic.

I would try soapy water to try and discourage black fly, I have found white fly on my petunia plants and have sprayed them with soapy water (the plant leaves are very sticky to touch). So far so good!

My dahlias didn't get the chance to attract blackfly. I nurtured them for weeks and the moment I put them outside to harden off they were stripped to the stalk by slugs/snails!
Sorry, I can't advise on your problem, hope you resolve it.

I would blast them with the hose and hope to get rid of the most of the black flies, if you do this the birds might clean up the remainder
Some people recommend spraying with a solution os soapy water