would I be correct in assuming that there are similar if not identical commands in C++? Also, can you keep the same simplistic style if you want to, like having just variables then creation code then functions, or does it have to have complicated syntax in order to work?
C++ is irrevocably mid-level, meaning that you have to be explicit about your usage of pointers and get used to the de-reference operator. You're protected from having to think about this stuff in the Blitz languages so switching over will inevitably take a bit of readjustment.
In terms of program-flow and the sort of data collections you'll typically use, however, there's a good deal of parity; Whether or not the more advanced, say, 3D-oriented commands are familiar or not will depend on the library/engine you adopt (for example I find Irrlicht a bit weird coming from B3D/MiniB3D because it seems obsessed with nodes).
Another thing I've been wondering is, someone told me it's not that hard to port a Blitz game to another language
Logic is logic and you'll find yourself expressing the same old stuff in most languages, only with whatever syntax they happen to favour. To this end, some sort of script-based solution (ie Unity, PyGame, Shiva or the like) might provide for an easier transition -- there's always going to be middleware and interacting with it at a high level is always going to amount to about the same thing. The stuff you learn with Blitz is eminently transferable.