This is an archive related to a previous version of Esri Maps for SharePoint. If you need the current version go to http://doc.arcgis.com/en/maps-for-sharepoint/.
The ArcGIS Map Web Part allows you to configure the transparency of each layer in the map. This allows you to see more or less of certain layers so you can emphasize specific data. Transparency can be configured for all layers, regardless of the layer's data source, including the basemap.