Is there a blank basemap available in ArcGIS Online?

Answer

No, there is no blank basemap available in ArcGIS Online. All web maps in ArcGIS Online require a basemap by design.

However, users have the option to set the transparency levels on the Esri's default basemap to 100% (transparent).

Another alternative is to create a blank basemap and add the blank basemap to the Basemap Gallery. Refer to the following knowledgebase article for more details: HowTo: Create a blank basemap for ArcGIS Online.