The x86 has had a HLT instruction ever since 8088.

It simply makes the CPU to pause or stop execution, until the CPU is reset or there is an interrupt request - which of course requires that interrupts are left enabled on the CPU and interrupt controller, and some peripheral is actually requesting an interrupt (such as system timer). So if you disable interrupts with CLI before running HLT, the CPU will never come out of halt state unless you push the reset button or turn it off.

Having a CPU instruction to halt display generation is impossible, PC video adapters keep doing what they were doing unless they are programmed to do something else like not to generate video, but that won't be a single CPU instruction, simply IO register writes like you normally would write video card registers.

TVs also did really turn off instead of just going blank. When turned off, or in standby, there will be no power on CRT driving circuits, i.e. the electron gun and deflection coils. It would make no sense to keep scanning the display with black lines when it is supposed to be off.