A custom file format can be a good idea if your application has features which you want to store that wouldn't be supported by any other formats, such as photoshop saving a file that preserves all the layers and information etc. Also if your application intends to be successful and widely used, a custom format is a good idea as it will then be adopted by other people to support files of that kind. After all, before photoshop there were no photoshop files.
I don't think Blitz is necessarily just a game programming language, I think that's just what it is geared towards and is mostly used for. If it were impossible to write anything other than games in it, I would call it a game language. But it's not exclusive. I am also working on something along the lines of a paint program in BlitzMax.
You could maybe look into using the existing C code of those file loaders and savers, but change them in some way to get access to the data in the way that you want to. Also maybe look around for existing libraries of file-format support that you could translate.
I don't think Blitz is necessarily just a game programming language, I think that's just what it is geared towards and is mostly used for. If it were impossible to write anything other than games in it, I would call it a game language. But it's not exclusive. I am also working on something along the lines of a paint program in BlitzMax.
You could maybe look into using the existing C code of those file loaders and savers, but change them in some way to get access to the data in the way that you want to. Also maybe look around for existing libraries of file-format support that you could translate.