ArcMap does not support blending modes. This is a feature found in ArcGIS Pro starting with version 2.7.
ArcGIS Pro Release Notes: https://pro.arcgis.com/en/pro-app/2.7/get-started/whats-new-in-arcgis-pro.htm#GUID-2F3B2C01-76F7-4AE5-B897-5E859EACBA4D
