Thanks a lot for the suggestion. The negative word actually worked. I had to use -15px instead of -10px which was still leaving a small blank space between blocks.

I used padding-top and padding-bottom instead of just padding because it was removing the left and right padding.

Here is the final code:

.bx-page-block-container {
margin-top: -15px;
margin-bottom: -15px;
}

Also I had to remove another css class from the same page that I previously added. It was conflicting with the .bx-page-block-container class. That's also the reason why .bx-page-block-container didn't work before. Thanks for helping. Issue fixed!