If the mint leaves have turned black after being frozen and then refrigerated, it is likely that they have undergone significant changes in texture, color, and flavor. While it's difficult to determine the exact condition of the mint without examining it in person, the blackened appearance suggests that the leaves have oxidized and potentially spoiled.
Boiling the blackened mint leaves to make tea may not yield desirable results. The oxidation process can lead to a loss of flavor and aroma, and the resulting tea might have an off-putting taste. Additionally, if the leaves have spoiled, boiling them will not make them safe for consumption.
To ensure the quality and safety of your tea, it's generally best to use fresh, undamaged mint leaves. If you're unsure about the condition of the blackened mint, it's advisable to discard it and use fresh mint leaves instead.