Like Tea, black coffee is permissible during Exodus 90. These 90 days provide you an opportunity to abstain from milk, sweeteners, or other additives in your coffee. Just black coffee is permitted.
Are you dependent on coffee?
If you find that you are dependent on coffee we would encourage you to utilize the graces of Exodus 90 to become free from coffee. Depending on your attachment consider some advice from men who have done Exodus 90 more than once.
Past Exodus men have seen Exodus 90 as a spring board, a time of preparation, for even greater freedom in the future. For example, a man who wanted to quit smoking took on Exodus 90 as it is and didn't worry about smoking during his ninety days. Then on Day 91 he channeled the graces he had received and the discipline he had grown in during Exodus 90, to quit smoking in Day 91. Perfect or imperfect at first, the by God's grace this man is free from smoking today.
If you are very attached to coffee, consider just focusing on the disciplines of Exodus 90 during your exodus, only having black coffee. Then in Day 91, use these graces of greater freedom to lean on God for even further freedom in Day 91. With God's grace you can be free from dependency on coffee.