A distro may be entitled to assume that it's just the system compiler that is in use, and provide a blabla.bmi in a compiler specific directory. Note that it can be required that the BMI for a module match the module name. However, because they are fragile, it does mean that libblabla-dev does have to ship all of the module interface files, just as it does for headers, so that the primary module interface can be rebuilt. 
In fact, that might be a requirement in any case, because if the module interface #includes anything, it's likely that the BMI will need to be rebuilt if the include file is changed. Although the dpkg build process should keep that up to date? It's definitely a build system issue, to the extent that you want or need to be able to have any shadowing of dependencies, where there's a source build of a library and binary inclusion of depender libraries building up into an application. I know that's fragile, but it does happen.