You can't actually make the legend patches transparent in ArcMap, because they ate basically graphics and transparency for graphics is not supported for a number of reasons.
You can try to workaround that limitation by constructing a legend out of features in a data frame.
ArcGIS Pro has more options for transparent colors throughout the software. And if a layer has transparent symbology the legend picks that up by default, as transparency rather than lightening the color. That's one of the advantages of the new architecture.