Some bloogers tell you to focus; others tell you to create as many blogs as you can. They all alert about blogs without focus as something evil, or, at least, not profitable.
Fine, but if my Blog focus on toothbrushes for carving mooses, and I want to talk about movies I watch, what do I do?
There are two possibilities:
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Loose the focus and add the off-topic post
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Create a new Blog
The first one is not a sin, if it happens once in a while, but if it becomes a habit, you’re doomed, your loyal readers will be upset about so many “useless” posts.
The second one only works if you’re willing to take care of another Blog and can generate enough content to make it stand by itself. Otherwise, forget the whole thing, and don’t post the off-topic at all.
People like me, who crave ADHS as a mutant power and call Ritalin “Kryptonite”, usually can’t live with a single Blog. I have my personal one, as generic as it can get a professional one with focus on ProBlogging and two English-language ones, Jungle Book itself and the Galactic Waste of Time, about humor ad sci-fi.
The last one is a bit undervisited (and underupdated), I confess, but it depends a lot of my mood. The others are more professionally run.
Can everyone do it? I doubt. There’s not enough time. I’m not even near my limit, but I have mutant superpowers, you know 
My advice is: Find the absolute maximum of blogs you can post/manage and spread the subjects you crave among them. Try not to loose focus. Create a generic one so you don’t pollute the others and STOP when you reach your limit. A post in a generic Blog is better than a post in a Blog updated once a year, that NOBODY visits.
And NO, I was NOT bit by a radioactive blogger. I was born this way.