The color of blocked dates and their hover message tell you why they are unavailable.
Advance-notice rule. Dates blocked by the advance-notice rule appear grey and show your custom tooltip message on hover.
Existing reservation. Dates blocked by a booking appear pink and show the standard unavailable message on hover. A run of unavailable dates that ends in a half-shaded day is a reservation. The half-shaded day is that booking’s check-out date and is still selectable as a check-out.