I just isolated a bug in the ABB calendar and have reported it to them. It's one that you might well have run into yourself and have it bite you in the rear. Please try these steps and let me know if it's a problem on your calendar, too.
Say you need to block off several consecutive dates. Edit your calendar and click on any open date to bring up the dialog with Available vs. Blocked. Click on Blocked. Then because you need to block off several days, click on the ending date and select (say) the day after. Note that the ending date changes as desired, but now THE ACTION HAS CHANGED FROM BLOCKED TO AVAILABLE. You might well not notice this, click on Save Changes, and move on thinking you had Blocked the dates.
I have been bitten by this bug and have paid the price of a conflict when a guest has booked the dates I thought I had blocked, all the while thinking it was my fault!