As far as I know Aquila do not sell a black string, they sell some darker ones (Lava) but not black. My suspicion is that the Uke was older stock and fitted with (once standard supply) generic black nylon strings, usually such strings work but not particularly well. Nylon strings can work well on Ukes but such (nylon) string selection isn’t straight forward, it’s something that’s best left to those that really understand things much better than most of us go.
I suggest the Martins M600 and the Aquila Super Nylgut as, respectively, good first and second choices. (Similar to the D’Addario variant there is a Martin Premium string too and IIRC that’s also made in partnership with Aquila ...)
Your Uke might require setting-up, if you want to then there are lots of old threads on that topic already on this forum for you to research.