What is the black spot in a tooth with sealant?

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Yes, your daughter has deeply grooved teeth (attachment removed to protect patient identity). By book, its treatment is in the following steps. Cleaning, that your dentist did perfectly as there are no residues of plaque or tartar at tooth surfaces. Deep fissure sealants, that is done to seal any carious activity if could occur in deep grooves. It is also done by your dentist. After the sealant treatment if any dark spots mean carious activity is seen it means that through micro cracks bacteria has traveled inside the enamel causing carious activity to start. Now, this is treated with fillings. It does not happen in a day or two, it takes like six months to cause slight decay in the tooth. In your daughter's case, this activity might have appeared after a few months.

Now your dentist will check if there is a catch in these black spots (carious cavity), then fillings will be done. Sometimes, there is no catch in these black spots, means carious activity is arrested and not propagating inside the tooth and they are not harmful to teeth and can be left untreated or just sealants are done to arrest them. If the black spots are becoming more evident, propagating to inside layers of teeth (harmful) and now some ditch or catch is formed in those grooves it definitely needs filling.