In order to select multiple blocked dates from the All Blocked Date list and disable the selected ones, go to Tasks -> Maintenance -> Blocked Dates, and select the multiple blocked dates you wish to disable. Then, click on the three dots on the top right of the list for Mass Action.
A pop-up window appears:
When you click on Yes, Do it, all selected blocked dates will be disabled and a Yay message on the bottom right corner of the page shows up:
Note: If you wish to disable one blocked date, tick the box next to the entry of your choice, click on the three dots on the right of the row and choose Disable.