CloudBlobContainer container = blobClient.GetContainerReference("files");
var documentID = "DCEE0856-D1F9-47B1-995A-CFA5AA999AE5";

foreach (var item in container.ListBlobs(prefix:null, useFlatBlobListing:true))
{
    CloudBlockBlob blob = (CloudBlockBlob)item;
    blob.FetchAttributes();
    if (blob.Metadata.Contains(new KeyValuePair<string, string>("documentid", documentID.ToString())))
    {
        break;
    }
}