Black GHS strings are likely the old-standby of 'tynex' similar to the original Dupont nylon strings of yore...
In my humble experience they have had a sort of dull tone and very little sustain, whereas I prefer the exact opposite sound.
I tried them and did not like them, it sounded like my uke had just died, totally lifeless.
You will get more improvement in clarity, tone, dynamic range and sustain with nearly ANY fluorocarbon strings compared to tynex, however some folks like the sound of tynex/nylon, and in that case, to each his own.
Martin M600 and M620 strings are a good choice as a starting point as has been said above, and are ~$5 per set like everywhere. I have Martin strings on several different ukes and like them very much on those ukes.