The main goal of Go Cloud is that the abstraction is not leaky and that you don't get access to specific features in the way you are describing in the application itself.
Now if the specificity of the thing you want happens at provisioning time but the API is the same (e.g. choosing to use Aurora on AWS vs standard RDS), then there's no problem. Go Cloud doesn't need to know about that.
If the API changes too, then that's a much different thing, and gets back to the leaky abstraction which we want to avoid. If you can't simply wire new providers to your application code and have it Just Work, then we've done our job wrong.
But remember that it's all Just Normal Go, there's no magic here. It's just standard Go. If you want to write an app that ties itself to something specific on some cloud you can; just write the code you'd write now. But that necessarily means you're tied to that product.
Contributors could also add drivers that not only fulfill your Blob interface but add features that might creep into multiple services. For example, someone could contribute a SuperBlob that fulfills Blob and also adds conditional writes, which GCS supports. On the app side, if they want to check if some feature is available that speeds up operations, they could then see if the returned Blob is also a SuperBlob and if so, perform the more optimized approach.
The main goal of Go Cloud is that the abstraction is not leaky and that you don't get access to specific features in the way you are describing in the application itself.
Now if the specificity of the thing you want happens at provisioning time but the API is the same (e.g. choosing to use Aurora on AWS vs standard RDS), then there's no problem. Go Cloud doesn't need to know about that.
If the API changes too, then that's a much different thing, and gets back to the leaky abstraction which we want to avoid. If you can't simply wire new providers to your application code and have it Just Work, then we've done our job wrong.
But remember that it's all Just Normal Go, there's no magic here. It's just standard Go. If you want to write an app that ties itself to something specific on some cloud you can; just write the code you'd write now. But that necessarily means you're tied to that product.