Normal company: Work Monday to Saturday, Sunday off. Black company: two Sundays a month off... but you have to work on them anyhow
Normal company: 70 hour workweek involving 20+ hours of paid overtime. Black company: the overtime is uncompensated ("service" overtime)/
Normal company: underperforming employees are ostracized or criticized. Black company: underperforming employees, or performing employees, are subjected to "power harassment" or physical violence.
Incidentally, salaryman loyalty counsels that one's own company is never a black company... I mean sure, there might be some disreputable firms in Tokyo, but we are just going through a crunch period, Tanaka-buchou is well-known to be demanding, that project is over budget so sacrifices must be made, etc.
Many Japanese salarymen of my acquaintance would identify the core underlying sin as "failing to uphold the company's end of the bargain all of us know we're making" and would say "It's mostly a thing at tiny SMBs who don't have the economic resources to consistently treat employees correctly."
If it isn't completely obvious, I have strong reservations about salarymanhood as a system and how the discourse of "black companies" is designed to and functions as insulation for that system.