I assume that you tasted the blackcurrents, I have always found them to be quite sweet when ripe. It could be that the gin which is designed to be quite bitter has dominated the blackcurrants. Seems a bit of a shame as the gin manufacturer added about 10 different tastes to produce it. Did you put the blackcurrents in mashed or whole. Sounds to me like you needed more blackcurents or perhaps some blackberries to help with balance. You can always take a small amount of your brew and add measured amounts of sweetener (low cal) and see if this improves the taste. If it does then add in proportion to the whole brew. Or as I have said before you can mix it with something else you have which you found too sweet, or just store it.