You have the exact opposite hair growth pattern to me.
My hair grows straight out of my scalp and naturally sticks out, where as the hair on my piece tend to lay flat at the back and again blending can be difficult.

The hair from your topper you get to stick out, but your natural hair is growing down wards and flat to your head.
As others have said, if you grew your hair a bit longer, just at the meeting point, you could brush it upwards and it would help it blend.

The blunt line you're currently getting doesn't look unnatural, but you could fade it better using a razor comb to just break up the ends & thin it out.

I've done a quick illustration to show how and where a bit of extra length at the back and brushing it upwards will help your hair blend in to the piece and gives your head a better shape too.
Wearing the cap won't help because the back of it is pushing the hair flat down.