In Strategy Web 10.5 and above, it is possible to hide the black toolbar for Visual Insight using below parameter in URL API:
hiddensections=dockTop
http://<web server>/MicroStrategy/asp/Main.aspx?evt=3186&src=Main.aspx.3186&subscriptionID=6515878E484686B953D6BCAC5FA5F57B&Server=ServerName&Project=Strategy%20Tutorial&Port=0&share=1&hiddensections=header,path,dockTop,dockLeft,footer

In Strategy Web 10.5, it looks like below when running above URL, note the toolbar is hidden:
