[Matt Biondi]

If the transmission oil turned completely black after nine months, it’s likely due to not changing the filter, an improper flush, or significant transmission wear at 120,000 miles. A clogged filter can cause contaminants to circulate, and if the mechanic didn’t properly flush the old fluid, it could have mixed with the new one, darkening it quickly. New transmission fluid should not turn black so soon, suggesting either the mechanic didn’t do a thorough flush or the transmission is experiencing significant wear.