Anodizing aluminum is not always perfect process and sometimes the extrusions will have anomalies like a white calcium build up from the wash tank or gummy tack from packing tape etc... stuck to the surface. Also from my experience black extrusions (from shipping or even just stacking them in the warehouse) can have what looks to be scrapes through the anodizing but turn out to be aluminum surface 'rubs' that leave a silver looking streak. Although these makings and discolorations...
Cleaning Aluminum Extrusions
Cleaning Aluminum Extrusions