How to Use Blend Tool (shortcut "W") in Illustrator Without Getting Blend in 1 Side?


Hello I wonder how to use Blend Tool (shortcut "W") without getting black like in the picture.
So I try to use the Blend Tool by clicking on the red first and then the yellow.
But the result is the red turn to black?
The questions are:
1. How to use Blend Tool (shortcut "W")?
2. Why did my object turn to black?
