Sidebar background automatically resize to size of blog post

Here is my blog: https://www.gillespiehall.com/blog/

The bottom of my sidebar disappears when the blogposts are too long – it just shows the black background. Is there any way to automatically “fill” the size with a color to make the page look even based on the length of the content next to it? I even tried doing one blogpost per page, but if the blogpost isn’t the EXACT correct length, the white body background will end and show the black background through, again, creating an uneven look.