Amplifier speaker outputs: is the black always the chassis?

^{simulate this circuit – Schematic created using CircuitLab}

Figure 1. Two common amplifier arrangements.

Can I safely assume this or are there any common or likely amplifier designs in which the black terminals might not all be at the same potential?

Bridge mode amplifiers use two amplifiers - one an inverted version of the other. The advantage is that on the same supply voltage you can now get double the peak to peak voltage. This is common in car audio systems where the supply voltage is fixed at 12 V nominal, but it is increasingly common in domestic amplifiers also.

The result is that you can't assume that black is common ground. Investigation is required.

Can I safely assume this or are there any common or likely amplifier designs in which the black terminals might not all be at the same potential?

In many amplifier designs (especially the power-efficient and well-integrateable and hence popular class D driving a H-bridge), both speaker conductors might alternate between any voltages within the range of the internal power supply.

Especially in stereo applications that means that left and right "black" can't be at the same potential for every point in time.

I don't know if there is some sort of home-entertainment industry rule, but it certainly is very common practice. NOTE that the black terminal is the reference potential ("system ground" or "signal ground"), but not necessarily connected directly to the chassis. The system ground might be floating with respect to earth ground.

Separate from that, be wary of class D amplifiers or linear amplifiers with a BTL (bridge-tied load) topology. In these cases, the black terminal is not connected to the system ground - it has 1/2 of the output audio signal on it, and the signals are different for each channel, so the black terminals are not interchangeable. Red and black still indicate correct phasing.