blendMode property Null safety

The blend mode to use to apply the filtered background content onto the background surface.

The only value for blendMode that is supported on all platforms is BlendMode.srcOver which works well for most scenes. But that value may produce surprising results when a parent of the BackdropFilter uses a temporary buffer, or save layer, as does an Opacity widget. In that situation, a value of BlendMode.src can produce more pleasing results, but at the cost of incompatibility with some platforms, most notably the html renderer for web applications.