A guitar is not a weapon. Even if an item is not a firearm, there may be laws regarding carry--if it is a weapon. In fact, since firearms are what most people (including legislators and judges) think of in conjunction with the word "arms" in the 2A, many States have laws against various forms of carry of various weapons that aren't firearms--laws that would be immediately tossed as unconstitutional if applied to firearms. In fact, in States with preemption, you can be fairly certain of gun laws throughout the State, but not so with weapons that are not firearms! So, watch out for local laws. You may be behaving perfectly legally, cross an imaginary line, and become a criminal!
For example, in Ohio, if I have any blade in my pocket, regardless of how short, I am carrying a concealed weapon--which is illegal, even with a license. Make it a gun, and now it is legal with that license, in every jurisdiction in the State.
My point is, do not accept what anyone else says the law is regarding black powder pistols in Texas. Folks who live in Texas can help guide you through Texas law on weapons and firearms, but make sure you read it for yourself.
If I had to guess, and I freely admit that I know zero about Texas law, carrying a black powder gun openly would likely violate either firearms laws in specific or weapons laws in general. To determine which laws would apply, look for the sections of the code that define the terms "firearm" and "weapon."
Now, just a bit of opinion: Why don't the courts consider knives as "arms" under the 2A. Why can States make virtually any law they wish regarding knives when the same laws would be unconstitutional regarding firearms. It should not be that way.