That is either the feathering of the area you painted or the anti aliasing. About the only way to minimize it with the brush tool is to set the feathering to 0. This will not completely eliminate it. You can try using tje pencil tool, but your edges will be very jagged.
Thats normal because without that "blending" you will see jagged edges when drawing curved shapes. Try with straight lines or draw shape with straight lines and you will not have "blending" which is actually mix of semi transparent pixels on curved edge of new shape with pixels below.
In order to see circle with smooth edges application will draw curved line which usually divide pixel(s). Because pixel can have only one color, pixel can not be divided or filled half or third using one color and remaining part using second color, the solution is to fill pixel using mixture of colors. That is case when you paint directly on the same layer. If you paint on separate layer try to hide layer(s) below then heavily zoom in to see what I am talking about and what is happening. In that case pixels on the edge where curved line is crossing will be semi-transparent. When you turn layer below again on, colors on layers above and below will visually mix and you will see "blending".