Battara,
No need for apologies. I have seen the same illusion myself, in photos and in reality under certain light
Maybe because of the contrast with niello.
Someone told me that niello over flat silver may peel away, that's why it is more common the combination with chasing. Is this true?
In this particular scabbard niello is on flat silver and it looks high polished. The chassing in the middle is plain silver.