Blackcurrants have the potential to deteriorate through the accumulation of viruses, almost always
Reversion Virus, which is easy to spot as it changes the
shape of the leaves. There's also Big Bud Mite. The remedy to both is the same, which is to replace the bushes. However, it's hard to avoid your replacements becoming re-infected / infested from neighbouring bushes. On my local allotments, nearly every black currant bush has Big Bud, and most have "reverted", but they remain, which in my view is irresponsible. Many are also hindered by some truly cretinous pruning — this also seems to be infectious.
Regarding netting, I've never netted my fruit, wherever I have grown it. Some may get eaten by birds, but not so many as to make a significant dent. A lot of attention is given to often over-engineered fruit cages (trellises for bindweed, sometimes) at the expense of the bushes therein. The way I see it, vigorous bushes properly tended yield plentiful fruit, plenty to go round.
Allotment, but little achieved.
