If the Add Layer List: Expanded option is selected in the Appearance tab there's no way to hide the legend.
If the Add Layer List: Collapsed option is selected the legend is can be collapsed or expanded, but it begins collapsed into the small icon and many users don't realize there's a legend available under the icon. It would be great to have the Expanded legend have a Collapse icon (perhaps an roll-up arrow in the legend title bar).
Alternately, the Collapsed legend could appear at first and then collapse itself after a few gestures which would draw attention to the presence of the legend and the Expand icon.