@andy, what you suggested makes sense. I simply restored to Default because it was there and never bothered changing it, in fear of messing things up. I simply installed Grav, got somewhat familiar and started using it. I'm learning as these issues arise.
I changed all my blog posts to Item and it makes a lot more sense now, but Grav did something weird where it updated the publish dates of some of my posts.
Anyway, then I tried again to use related pages. I put it in both my sidebar and blog_item. Sadly, no luck. Related pages was not in the sidebar by default for me. I then went on and installed Random Article plugin and it's working just fine. It was there in my sidebar by default. Also, random article didn't work right away, I had to change the AND operator to OR.
The point in saying all of this is that, I am definitely missing something but I don't know what. If you outline exact steps, directories that worked for you, it'd be more helpful for me. Maybe even your exact settings and configurations for your Grav.
Really appreciate the help. Hopefully these problems won't arise as I get more familiar with Grav. I have been reading Grav documentation from the very start, it's just new territory.