hmmm – no expert but as someone operating a complicated netting this and take the risk with that system I have always taken the risk with blackcurrants on the condition there are no bullfinches around. I have no idea what a bullfinch looks like so I continue to relax and mindfully leave my bc to fruit as they choose.
I recall the advice as bullfinches love bc buds but a quick scurry through the authors most likely to offer that sort of writing has not produced the writing in my book collection. If I find it I will post.
If the damage looks insect driven try straying the bush with soapy water. If it is something that looks as if it might spread prune the affected branches out and bin them before it rains -or you spray has just suggested in case a virus spreads. I am seeing pigeons and blackbirds in my orchard garden at the moment but no damage to leaves. I do not know if there is a blackcurrant equivilent to the gooseberry sawfly but as you say there is no sign of caterpillars/moths/larvae that does not seem to be your problem.