How to infuse black layer of peppercorn into an oil?
If you insist on an oil, it can be done by taking black peppercorns, infusing them in high-grade alcohol for a couple of weeks at room temperature and then evaporating 90% of the alcohol and infuse the last 10% of the alcohol into the oil.

The easy way would be to infuse them in the cheapest moon shine vodka you can lay your hands on for 3 months and then using the vodka $instead of oil.*

If you insist on only the outer part, you can use an electric potato grater to peel off the outer skin only and infuse the "black powder" :-) in a neutral oil like grapeseed oil heated to below its smoke point for as long as possible.