Black salt or kala namak is actually great for mild acidity and nausea during pregnancy. But yes, too much sodium is not good, so don't make it a daily habit. A pinch here and there is perfectly safe.
No, it's not bad at all if you use it in normal food limits. It actually helps with digestion when you feel bloated. Just don't eat it excessively on raw fruits every day.
A little bit of black salt in your nimbu paani or salad is fine, but don't overdo it. My gynac always told me to watch my salt intake because it can affect blood pressure and cause bloating. Just use it in moderation.
With my twins, I was constantly bloated and black salt in chaat masala was my best friend! Just don't go crazy with it because high BP is no joke during pregnancy. Keep it to a pinch!