If, as you suggest, the blank page is not officially required if you already have your visa, then as long as you had proper documentation and presented yourself at check-in at least by the check-in deadline specified in your itinerary/communication from the airline or travel agent (or, if no such deadline was specified at least 45 minutes before departure), and were refused boarding, you have been involuntarily denied boarding within the meaning of Regulation 261/2004 and are therefore eligible for compensation.