Dark rough oxidized casting problem is a hard black dust coat on the casting due to heavy oxidation during melting and can occur to non-deoxidized alloy.
Please check:

1. Flask overheated during burn out cycle, check oven temperature.
2. Metal overheated, check thermocouple or overused.
3. Oxidizing flux used on metal before casting, such as flux for refining.
4. Used non-deoxidized alloy.