It is blocked because it is contained in one of your subscribed block lists. If you query your gravity list it will show this domain is contained as shown below. That list is one of the seven lists you are offered when you install Pi-Hole. The maintainer of that list likely determined that this domain was serving up ad content.
Match found in https://raw.githubusercontent.com/StevenBlack/hosts/master/hosts:
That is your call. It’s probably not "unsafe,’ but may serve up ads. If you want the content, then whitelist the domain. If you whitelist and don’t like the content you receive, then remove it from the whitelist.
Typically a list maintainer will have a feedback mechanism, where subscribers can contact them with recommended corrections.
Edit - I tested https://www.engadget.com and whitelisted o.aolcdn.com and didn’t see any ads, just the images for the page.