By clicking the blocked time on your calendar, you’ll bring up a window with an option to delete. If it’s a recurring block, you can even choose between deleting the individual block or the whole series. Check it out:
If the time is blocked on multiple calendars, you can unblock it on a specific calendar by clicking the block of time on said calendar and removing it, OR by removing the time from the Availability area of your calendar (in the Blocked Off Time section). Here’s an example...
