I have a BlobStream that is created from the method OpenWriter.
var blob = CloudContainer.GetBlobReference(name));
if (blob == null)
You can seek within a page blob - there is explicit support for it in BlobWriteStreamBase class.
I think you could also read & write to specified parts of a Block blob using HTTP
Range headers, which would be effectively the same thing as seeking. But I think you'd have to implement that yourself.