CloudBlob.Metadata Property

static void WriteBlobMetadata(Uri blobEndpoint, string accountName, string accountKey)
{
    // Create service client for credentialed access to the Blob service.
    CloudBlobClient blobClient = 
        new CloudBlobClient(blobEndpoint, new StorageCredentialsAccountAndKey(accountName, accountKey));

    // Return a blob reference.
    CloudBlob blob = new CloudBlob("mycontainer/myblob.txt", blobClient);

    // Write some text to the blob.
    blob.UploadText("this is a text blob");

    // Define some metadata for the blob.
    blob.Metadata["category"] = "images";
    blob.Metadata["owner"] = "azureix";

    // Write the metadata to the service.
    blob.SetMetadata();
}