If a black hole is truly black and has an escape velocity greater than the speed of light such that no light can escape it, where do the X-rays come from in the black hole candidates so far identified?

The X - rays are not emitted by the black hole itself but by the from hot gas orbiting around the black hole forming a disc known as accretion disk. When a black hole rips apart a star it attracts gases towards it. The pressure that is generated in the gas heats it up to millions of degrees thus emitting radiation among which x rays are also present which are detected by radio telescopes on the earth.