Robert Cartaino's answer, for why the "blatantly off-topic" sub-reason* of the off-topic close reason used to exist and why it doesn't anymore, can't be beat. But as for the specific question you had chosen "Other" for and written a custom comment, I think you may be wondering which of the stock sub-reasons you should've used instead.
For questions that have nothing to do with Ubuntu at all, just use the first selection under off-topic: This is not about Ubuntu. Although the message links to (currently four) other Stack Exchange sites, it applies even to questions that would be off-topic on those sites as well.
Because of its bolded opening text--"This is not about Ubuntu"--it is usually understood. But if I'm worried it might not be understood, I sometimes also write a comment like "This has nothing to do with Ubuntu," or, "You've asked this on Ask Ubuntu, but this question doesn't have anything to do with Ubuntu, does it?" Such comments aren't really necessary, though, and they certainly needn't be posted as a custom off-topic sub-reason.
Sometimes there is a hidden connection of some sort but the question is unclear or needs to be asked in a radically different way. So if you want to say something special, a better thing to say might really be:
Is this [related to Ubuntu](https://askubuntu.com/help/on-topic) in some way? If so, please **** to clarify.
That looks like this (except that the system will automatically make edit into a link that edits the post):
Is this related to Ubuntu in some way? If so, please edit to clarify.
In any case, that would be reasonable as a regular comment, but I still would not recommend putting something like that into the close dialog.
Still, I don't think it's a big problem that you used a custom sub-reason. Either way, it was going to be closed as off-topic, and indeed, it reads as being just about as blatantly off-topic as any question could be. Using a custom sub-reason needlessly is not something that should be encouraged, but you're still closing the question or (for users with less than 3000 rep) flagging for closure and the reason you have is not wrong.
The main problem is that the comment that gets posted in your name when you do it is pretty useless:
I'm voting to close this question as off-topic because this is blatantly off-topic.
To be useful, it would have to have said something like "not about Ubuntu" instead of "blatantly off-topic," so you can just pick that instead of clicking Other.
This is different from a custom moderator intervention flag (i.e., clicking in need of moderator intervention), which must always be tended to manually by a moderator. Those should be used when there's no other flag that does the job. When one uses a custom off-topic sub-reason unnecessarily, one is still using the regular procedure for closing questions.
* I assume there is some official term for this, but I don't know it. In the Stack Exchange Data Explorer, the close reason (like "unclear" or "off-topic") is represented numerically by the
CloseReasonTypeId, and what I am calling a "sub-reason" is represented by the