The toe pick also looks different. The toe pick is usually shorter on a dance blade, so that the tip is farther from the ice, but the angles that the toe pick meets the ice are much different. The combination has the effect of making dance blade toe picks much less effective, but also much less likely to cause you to stumble.
I think most synchro blades are a compromise - shorter tail, like a dance blade (some synchro blades have shorter tails than same brand dance blades, but that is not always true), but a toe pick that is intermediate between that of a dance and freestyle blade. Some people on this board prefer synchro blades, even when not doing synchro.
But there is sufficient variation in blade shape, from brand to brand and model to model, that it is difficult to generalize this way.
Finally, the most popular ice dance blades (at a competitive level MK Dance genuinely dominates) are also "thinline" - thinner than most figure skating blades, though they are ground that way only at the bottom, in the "Chrome relief zone", where the nickel/chrome plating is ground off. This is supposed to make them faster. I know for a fact that my MK Dance blades felt overwhelmingly faster than my Coronation Ace blades - and overwhelmingly faster than my Ultima Supreme freestyle blades as well. However, that may have had to do with the shortened toe pick too - because one could roll through more of the length of the blade.