Even though global blocks are shown as a widget, the block to add them does nothing. This is because the Widgets doesn't support "core/block" which is what is reported as an error. If you register the core/block, it does show though but is broken once loaded in. We are wanting to use this on a client project right now for A/B testing Ads for them. Also would be good to be able to create the Global block from inside the Widgets screen instead of elsewhere.