Download blob storage and return Json object

Please reset the stream's position to 0 after reading the blob into the stream. So your code would be:

        using (var stream = new MemoryStream())
        {
            blob.DownloadToStream(stream);
            stream.Position = 0;//resetting stream's position to 0
            var serializer = new JsonSerializer();

            using (var sr = new StreamReader(stream))
            {
                using (var jsonTextReader = new JsonTextReader(sr))
                {
                    var result = serializer.Deserialize(jsonTextReader);
                }
            }
        }

Both the question and accepted answer start by copying the entire stream into a MemoryStream which is effectively a big byte array in memory. This step is unnecessary - it's more memory-efficient to stream the blob data directly to the object without buffering the bytes first:

using (var stream = await blob.OpenReadAsync())
using (var sr = new StreamReader(stream))
using (var jr = new JsonTextReader(sr))
{
    result = JsonSerializer.CreateDefault().Deserialize<T>(jr);
}