Hi Sung,
We've finished our research about this issue. Unfortunately, I determined that this is not an SDK issue after all. It's a deeper issue- we don't support reading and writing blob fields from ArcGIS Server-based feature services. (Technically this isn't true, but the only blob support is within annotation features; generic blob fields are not supported). You cannot read or write blob fields with the Pro user interface, with Python, or with the Pro SDK.
Blob fields are supported in hosted feature services, in enterprise geodatabases, file geodatabases, and mobile geodatabases.
Sorry- this probably isn't the answer you wanted to hear.
--Rich