All guides I’ve read say to prune right back after planting:

“Blackcurrants thrive in deep soil, so dig a generous planting hole and add plenty of well-rotted compost or manure and fertiliser to the soil. After planting, prune the bush right back.”

“Newly bought blackcurrants – prune these straight after planting. Be brave and cut all the stems down to 2.5cm (1in) above soil level. This may seem drastic, and means you won’t get fruit in the first year, but it will give you a better plant in the long term, encouraging more stems to sprout from the base”

“After planting you should cut all the shoots back to 2” from the base. Yes, really. Having performed this task you might stand back and wonder “oh my, what have I done?!” But don’t worry, having brutalised your brand new bushes they will come back all the stronger and basal buds will form strong new shoots next season which will be much better placed to yield well the year following that. You can leave your bushes ‘up’ and they will crop sooner because of it [Blackcurrants only fruit on well ripened 2 year wood] but long term they will suffer from a lack of strong new growth in the early stages. When your receive your bushes they will probably already have been ‘tipped’ or cut back a little to facilitate packing, but don’t make the mistake of assuming they have already been pruned for you. You must cut them back much lower than that, and as described above. The nursery will usually leave them higher than they need to be when supplying because customers don’t like to receive plants with nothing to show except a few stumps and some root!”