I strongly advise you to seek a second opinion from another medical doctor to further assess and address your situation. Dark blood or black stools can sometimes indicate bleeding in the upper gastrointestinal tract. This could be a serious side effect or an indication of an underlying issue. Please ask your GD to guide you to a good medical doctor. Since you are pregnant, it's especially important to inform your healthcare provider about your situation, as certain medications may have implications for early pregnancy.
I highly recommend incorporating magnesium oxide/citrate (500mg) into your morning and evening routine, along with a supplement of highly concentrated Omega-3 fatty acids (3-4 capsules). This regimen not only aids in soothing your stomach but also provides potential relief from conditions such as heartburn and various other ailments in the long run. In managing heartburn, incorporating probiotics, aloe vera juice, peppermint tea (known for its anti-nausea and heartburn-relieving properties), and including bananas in your diet can also be very beneficial. These elements contribute to overall digestive well-being and may help alleviate symptoms of heartburn and possibly other ailments.