ArcGIS 10.2, ESRI Maps for MS Office Why a heatmap that was created in Excel and looks fine and is still fine on a preview of a map if I publish it it ArcGIS Online but turns into black points if I open it in ArcGIS Online map viewer ?
This is expected behavior. The ESRI Maps for Office help references this:
Heat map layers and layers that are clustered cannot be shared to ArcGIS Online as a layer, but can be shared as part of a map. In the ArcGIS.com Map Viewer, the heat map layer displays as a point layer instead of rendering as a heat map.