"Can" versus "should"...
If you create real blackwater, as opposed to just flinging in botanicals with no regard to mineral content, the answer is no. With almost no measurable hardness, swordtails will do poorly. Guppies survive in lots of different waters, but I noticed they were in coastal water in Africa, and thankfully hadn't survived their introduction to blackwater interior habitats.
It would be very physically stressful to the fish.
Blackwater is usually tannin loaded, but in the 50 or less tds zone. You won't have blackwater tanks at 120 tds. I had 140 tds for years, and for blackwater dwarf Cichlids, I needed to cut it by 2/3 with rainwater, snow melt or later, RO.