You asked about transitioning swordtails and guppies to true blackwater, which, true blackwater is acidic and low GH. Very low GH, often well below 120ppm. PH often below 7.
So, I'm the one being left confused here by what you are trying to get at?
Will swordtails and guppies do well in true blackwater? No, they will not thrive well lifelong due to the fact that true blackwater is low GH, low pH, and low TDS.
If you are simply staining the water without messing with your water parameters, then it's not 100% true blackwater in anything except aesthetics and this is a totally different question. However, someone had already asked about that and you had replied you were aiming for true blackwater. Hence leading to my current response.
I am lost as to what you are trying to get at here aside from what's been answered lol