The expressions ‘in the black’ and ‘in the red’ originated in the business world, and have been part of the language since the beginning of the twentieth century. When you say that the company you are working for is in the black, what you are suggesting is that the company is doing well; it is making a profit. The opposite of this expression is ‘in the red’. When a company is in the red, it is running at a loss; it is losing money.
We all know that in English, the colour ‘black’ is seldom associated with anything good. It is always associated with something negative. When you are angry about something and are in a foul mood, you are said to be in a ‘black mood’. Then you have words like ‘blackmail’, ‘black magic’, ‘blacklist’, ‘blackball’, etc. — all of which suggest that the colour is generally not associated with anything good. So, why is it that people are happy when they learn that their company is ‘in the black’?
It has to do with the way that ledgers were maintained in the past. For several centuries, accountants made use of two different types of ink when making an entry — black and red. All the credits (the money coming into the company) were written in black ink and the debits (the money going out of the company) were entered in red ink. So, if a ledger contained more entries in black ink than in red, it meant that the company had more money coming in than it had going out — the company was making money. The two expressions can be used with organisations and individuals. When you say you are in the black, it means that there is money in your bank account.