What is the origin of the phrase of in the black?

It is related to money and finances.

In the past, when books of account were written up, the entries were made in black ink. But negative balances were written with red ink. This practice is often continued today, whether the accounts are maintained manually by book-keepers, or processed on a computer.

The bank 'account', for example, shows the bank balance at the end of each day. If the balance goes into a negative figure, the figures were/are written in red, to draw attention to the adverse state of affairs. A company, or a private individual, who had 'gone into the red', or who were 'in the red' were in a state of financial loss. Conversely, if the bank balance was positive, they could be said to be 'in the black', which would be a good state to be in.

Being in the black is the opposite of being in the red. If a company is 'in the black' it is financially sound, and making a profit instead of a loss. ---- "TRADE TURNAROUND KEEPS AUSTALIA IN THE BLACK"

(News headline, June 3, 2009)

The column begins ... 'Australia'S economy expanded in the March quarter while the rest of the developed world slumped, figures showed yesterday,...' ~Western Australia Today. (WAtoday.au.com) ---- For more information, see Related links below.