What would happen to heart rate if some stimulus caused blood pressure decrease?

A decrease in blood pressure would cause an increase in heart rate.

The baroreflex is a homeostatic mechanism that helps maintain blood pressure within a narrow range. The baroreceptors are located in the carotid sinus and aortic arch, and they sense changes in blood pressure. When blood pressure decreases, the baroreceptors send signals to the medulla oblongata, which increases sympathetic activity and decreases parasympathetic activity. This causes the heart rate to increase and the blood vessels to constrict, which increases blood pressure.