To transfer an Azure blob via Rebex SFTP client to an SFTP server without the need to keep data of the whole blob in memory, try this approach:
// get blob container client
var container = new BlobContainerClient(connectionString, "...");
container.CreateIfNotExists(PublicAccessType.Blob);
// establish Rebex SFTP session
var sftp = new Sftp();
sftp.Connect(...);
sftp.Login(...)
// get block client
var blobClient = container.GetBlobClient("myfile.zip");
// transfer data from the blob to a file on an SFTP server
using (var uploadStream = sftp.GetUploadStream("myfile.zip"))
{
blobClient.DownloadTo(uploadStream);
}