Can you use the mana from Dark Ritual to pay for a commander tax?

Dark Ritual produces three black mana (BBB) that goes into your mana pool. Once mana is in your mana pool, it is simply mana — it has no restrictions based on where it came from, unless a card specifically says otherwise (CR 106.1).

The commander tax (formally called the 'commander alternative cost') is an additional cost of 2 generic mana for each previous time you've cast your commander from the command zone (CR 903.8). Generic mana costs can be paid with mana of any color or colorless mana (CR 107.3c), and colored costs can be paid with the matching color. Dark Ritual's black mana can pay for either the generic portion or any black pip costs in your commander's mana cost.

For example, say your commander is Kess, Dissident Mage (1UBR) and you've cast her once before. Her total cost is now 1UBR plus the 2 generic tax, totaling 1UBR+2. You can tap lands for most of it and use Dark Ritual's BBB to cover the B in her cost plus two of the generic mana, paying the full commander tax with no problem.

There is no rule in Commander or the Comprehensive Rules that restricts which mana sources can pay for the commander tax. As long as you produce enough mana of the right types to cover all costs, you're good to go.