Tooth extraction on thursday preceded by clindamycin 600 mg for premed before procedure. Regularly take miralax (polyethylene glycol) for constipation. Black loose stools?

Evaluation. Black stools indicate bleeding somewhere along GI tract. Single dose of antibiotic and the small amount of bleeding from a single tooth extraction is probably not the cause. Run your symptoms by your primary care practitioner. Further studies may be indicated.
Medication use . Taking some medications can cause GI changes. Contact your prescribing doctor if the problem continues.