Is black tea bad for pregnancy?

No in moderation, maybe if consumed excessively.

The main ingredient in tea that is of concern for pregnant women is caffeine.

The current medical consensus is that it is safe to consume less than 200mg of caffeine daily during pregnancy. There is no agreement about how much caffeine above this is too much--some studies have suggested increased risk of miscarriage and other complications with higher consumption. The 200mg is a safe limit--there is no evidence amounts below 200mg / day cause any problems with pregnancy.

The caffeine content of black tea varies widely, but generally ranges from 15-75mg per 8 oz. cup, depending on the type of black tea, how much leaf is used, and how long it is steeped. 2 or 3 cups of most black teas should be completely safe, and more cups of a low-caffeine black tea like Lapsang Souchong might also be safe. This stands in contrast to coffee which often contains much more caffeine--two stronger cups of coffee could put one over the 200mg limit.

It is probably best to space this caffeine throughout the day rather than all at once.