File name is not a blob because it is entered by the user as a text string and displayed for the user as a text string and not as a bunch of hex digits. Also, it cannot contain some characters (like slash or null) so it's not a blob anyway.

And you should be using one specified encoding for file names if you want them to be displayed correctly in all applications. It would be inconvenient if different applications stored file names in different encodings.

For the same reason, encoding should be specified in libraries documentation for all functions accepting or returning strings.