I think comments are nearly always enabled for the blog posts. So yes it is usually possible to make them. (But if you haven't made a comment before, they often need approving first.)
I can think of one or two exceptions where a post had comments deliberately disabled, but these are extremely rare.
As to your other thread (why are posts deleted?) I have deleted it because it was superfluous.
You posted this thread on Saturday at 1301. Moderators are volunteers who have lives. Your first post on the forum has to be approved, thereafter you can post freely (it keeps out a lot of robot spammers). I just approved this post 3.5 hours after you posted it. It was not deleted, it had merely not been approved yet. 3.5 hours is not that bad for a weekend.
We generally prefer not to delete posts or threads, but we do so when we feel is necessary for keeping the peace and keeping things tidy (e.g. when people start multiple threads not knowing why their first post hasn't been approved yet).
Alex Eames RasPi.TV, RasP.iO