type BlobStore interface { /* methods */ }

    // later, when initializing stuff
    var foo BlobStore
    switch {
        case isAzure:
            foo = AzureBlobStore{}
        default:
            foo = FSBlobStore{}
    }
    
    svc1 := NewSvc1(foo)

    // Constructors are just functions and are totally 
    // convention/arbitrary. If the fields are public you 
    // can instantiate the struct directly.
    svc2 := Svc2{ BlobStore: foo }