I have two ways of dealing with interruptions from my husband. The first is to plainly state "You've interrupted me. I'm not done what I'm saying." Which is blunt, but befitting the rudeness of being spoken over. The second is to gently stroke his cheek and say "Gosh. You're SO pretty". It's our code for shut up now
I don't know why this tickles me as much as it does.
Do you only do this when you're alone, or do you suddenly break out with "Gosh. You're SO pretty" in front of other people in the conversation. I'm just picturing a group conversation taking a sudden turn into the surreal when people see this for the first time, especially if you're discussing some serious and completely unromantic topic at the time.
To me, the OP sounds like a problem of different conversational styles clashing. I have plenty of friends with whom conversations go veering off on long tangents due to a comment on some detail. But it works because both people communicate in the same style. If one of us wants to complete the original train of thought, we either hold the thought and continue after the tangent conversation or pause the tangent with something like "Before I forget, the original point was..." I think establishing some signal or outright telling him when you're trying to make a point and he's interrupting would be good. However, if going off on tangents is his normal conversational style, then I would urge you to sometimes go with it when you're having a casual conversation and not trying to make an important point. It's important for him to respect your conversational style, but that goes both ways.
One thing I'd like to point out about your "blue skies" example (I know you just made it up, but you might want to think if actual instances are like this):
Your statement "Good thing it was blue skies and not raining" sounds like a conclusion to me. It sounds like you've finished your story of your difficult commute and are wrapping it up. So your BF's comment doesn't strike me an interruption. To me, it sounds like a way to continue the conversation based on a cool tidbit that your story brought to mind. "Speaking of [the phrase] 'blue skies,' did you know that they actually only look blue because..." So you might want to think about how you
would have expected the conversation to go. Did you actually have more to tell him about your commute, which he cut off? Did you want him to comment or commiserate before moving on with the conversation? Would a tangent on "why the sky looks blue" be OK if he hadn't phrased it as a correction ("the sky isn't actually blue...")? If you only tell your BF what you don't want him to do, he may still not know what you do