The difference between a black pewter and a silver streak is the presence of a second instance of the pastel gene. When the animal is a super pastel black pastel (e.g. silver streak) it has a distinct absence of pattern down the center of its back that is ...silver (which explains the name).
Here are three pictures. The first picture is the ghost lesser black pewter. Notice the absence of a streak down its back. The second picture is a regular silver streak (using the word "regular" to describe a silver streak is a bit of sacrilege). The third picture is this silver surfer. Notice the "silver streak".
Hope this helps.