Continuously attempting past papers under exam conditions certainly aids this process but you need to consider why your mind blanks out, because otherwise you're only practicing how to get 80% and not 90%. Of course, the alignment algorithm typically brings results of 80% up to E4 in MX1, but that's besides the point.
Firstly, make sure some past papers you do (in or out of exam conditions) are set by the same people as those of your school. Your school should be offering you some relevant past papers for you to do to get a feel for things. Under normal circumstances, your school will continue to set past papers from said company (e.g. your school writes their own, CSSA, blah) and you should do those as well, so that you're being exposed to questions much more like what you'll get on the day.
Blanking out isn't abnormal but you need to figure out why, as well as how to deal with it. When does the blanking out start - reading time/writing time/before you open the paper etc. What exactly happens - just stop thinking or what? Do you drink water in the exam? (Yes, water helps.)
Then, figure out ways of dealing with it. You need to stay calm, and employ basic exam strategies (such as getting rid of easy questions first). Maybe not necessarily force it, but you need to allow yourself to regroup as well.
Fully compare what happens when you do past papers, and in the exam. By full, I mean to every last detail you know of.