Hello,
Black stain could be due to many reasons- most common cause is staining due to iron supplements in your case, it could also be cavities or chromatic staining
Tooth brushing with normal water will not suffice, I would recommend using a toothpaste
Next Steps
You can consult with the clinic so we can see be sure of the exact reason for the staining and help you with it.
If it is due to the multivitamin, give water after it to avoid further staining. However current stain will need gentle professional cleaning.
Also, the first visit of the child to a dental clinic with a pediatric dentist should ideally be done by their first birthday, so that would be advised. We can guide you with building healthy habits that are easy to follow with such a small baby and that would go a long way
In person guidance can be given with exactly which toothpaste, brush, how to brush and future care for your child ☺️