Blank slate as in the front office can project whatever they want on to him. He has no preconceived managerial tendencies, therefore the assumption is he'll be open to learning and will manage the way the front office wants him too (using advanced metrics, applying situational leverage correctly, able to sell that to the players, etc.).
Which, obviously can be a good thing. Or you can do what Matheny, Ausmus (x2), Matt Williams, or any of the other recent managers with no/ not much managerial experience and just throw away the stat packs and do your own thing, especially panicking over small samples in the face of advanced metrics telling you to stick to the path. It happens a lot with recent former players turned-coaches, they surface level accept what the nerds in the front office are (correctly) telling them to do, but years of ingrained "baseball knowledge" take over and all the sudden you're shaking up the line to "get guys going" and starting Matt Adams in LF because "we need his bat in the line-up" and starting the utility infielders over the better players because they're "hot" or "have gone 5 for 9 against this pitcher historically, so he obviously has his number"