The big advantage of BlitzMax having clean, simple hooks to graphics, sound, I/O, and other useful system-level things like timers. And even more so, these are fully CROSS-PLATFORM hooks. On one particular OS, none of this is difficult. It can be finicky, yes. Sometimes you have to hunt down weird bits of knowledge (especially for things like switching to fullscreen mode), but it's all doable. But you will still only have covered one platform, not three. Maintaining cross-platform code to do all of these things is messy.
A secondary advantage is that Blitz is generally easier to mess about with than C and C++. Maybe not hugely so, but the advantage is still there. This comes from a lot of little things that have a cumulative effect. For example, there aren't endless compiler switches to mess with. You aren't as tempted to unnecessarily compress code to the point of being unreadable. You don't have to puzzle over obscure declarations as much.
Now, if your goal is to work directly with existing C code and structures, then I'm not sure what you'll gain with Blitz. You might be better off using a game library for C++. Occasionally converting C structures over to Blitz isn't so bad, however.
A secondary advantage is that Blitz is generally easier to mess about with than C and C++. Maybe not hugely so, but the advantage is still there. This comes from a lot of little things that have a cumulative effect. For example, there aren't endless compiler switches to mess with. You aren't as tempted to unnecessarily compress code to the point of being unreadable. You don't have to puzzle over obscure declarations as much.
Now, if your goal is to work directly with existing C code and structures, then I'm not sure what you'll gain with Blitz. You might be better off using a game library for C++. Occasionally converting C structures over to Blitz isn't so bad, however.