I don't think Derek is always wrong, but he may be mistaken on this score. I looked at all the planes I have, a 46, 45 and 55. They do indeed all have curved blades, and they are all curved the same way. That is, when you place the blade on the skate, there is a hollow in the center of the back side of the blade. As Steve suggests, when the tapered bolt that holds the cutter is tightened, the blade is flattened to the skate and held under tension. Interestingly, I've always thought that the belly was a product of heat treating the blades and not due to a deliberate design feature. Further, I have an early japanned number 50, and as most know it's blade holding mechanism is not the same as the others, in that the entire blade is clamped from both sides and held tight by a wing nut and bolt. That blade is absolutely flat. Very curious and great catch Steve.
The photo is below. There is an early japanned style 46 blade on the bottom of both photos. The upper photo has an early floral plated 45 cutter on top, and on the bottom photo is a 1900 era 55 cutter on top. Cutters placed to double the curve for clarity. (look at the bevels).
Attachment 364506
PS. I just watched the youtube video again, and it looks like the large plow has a pivoting lever cap sort of like the Stanley 78. As such, I'm not sure the curved blade matters, as it will be clamped near the edge where the cutting happens and at the top, making it pretty chatter free, just like the 78. An interesting observation on the older planes though, none the less.