My observation:
Five wires come out of the tonearm. Red is R+, green is R-, white is L+, blue is L-, and black is tonearm chassis/ground.
Black is not shorted to green or blue.
Black is usually connected to the turntable chassis and possibly the motor (at least I have done it on my Lenco), then finally connected to the earthing point of the phono preamp, through its dedicated wire.
Here, once it reaches the phono stage, things get a bit muddy. For example, on the CNC DIY phono stage, the ground "G" point of power supply (of +V/G/-V) is also the earthing point. There is no dedicated earth point. This, in my opinion, creates a ground loop.
A more purist approach would be to isolate the supply ground and equipment earth even in the phono stage, and then in the line level preamp too, and finally ground the earth that originated in the tonearm chassis to power ground, only at a single point, in the power amp.
If it is an integrated amp with no built in phonostage, grounding happens on the integrated and not on the phonostage.
If the integrated has a built-in phonostage, there will be a built in grounding terminal too, and this is the only point where one can ground it.
For example, on my Pearl II build, as advised by omishra I took great pains to avoid shorting the chassis to the RCA female socket -ve terminals, but IIRC, all that care is not possible on the CNC. But I don't have a CNC anymore to check, though.
Now, soldering the earth wire to one of the RCAs on the PCB, is equivalent to shorting black to green or blue.
There are turntables where this is done and so does not have a separate ground wire emanating from the TT. It can work, but it is subject to trying.
I would personally favour the purist approach.
