Can you post the photo here? It is normal that dark marks appear on the spindle where the bearings sit. Unless there is an extreme amount there is no concern. The condition is called fretting corrosion and on a trailer is caused by that small amount of clearance between the bearing and the spindle plus no grease on the spindle at assembly. Here is a minor
example. If you provide a photo, we may be able to save you all the expense.
The time to service trailer bearings is the minute you bring it home. Inadequate grease and poor adjustment happens all too often.