C'è un warning a proposito dei nomi dei font per quella funzione:

fontfile

The path to the TrueType font you wish to use.

Depending on which version of the GD library PHP is using, when fontfile does not begin with a leading / then .ttf will be appended to the filename and the library will attempt to search for that filename along a library-defined font path.

When using versions of the GD library lower than 2.0.18, a space character, rather than a semicolon, was used as the 'path separator' for different font files. Unintentional use of this feature will result in the warning message: Warning: Could not find/open font. For these affected versions, the only solution is moving the font to a path which does not contain spaces.

In many cases where a font resides in the same directory as the script using it the following trick will alleviate any include problems.
Potrebbe essere il tuo caso? Per prova, sposta il tuo "Arial Black.ttf" nella cartella dello script e prova a rinominarlo senza spazio per l'inclusione.