Just because it is an .apk, does not necessarily mean it will install. With that, if it does install, it may not work correctly either. Some .apk files (blur files for instance) have other dependencies such as the moto-res.apk or the blur-res.apk files that come with the stock Motorola ROM's. These additional apk's have the dependencies built into them that allow the blur components to run correctly. Without the necessary configuration (not exactly sure, as I have not done ROM development) these blur apks will not work.
My guess with blur apps such as the blur camera, is that they do not rely on the moto-res or blur-res files in order to run, or only need a modification to a lib file to work correctly. Again, I'm not a programmer, but have a decent amount of IT knowledge and background for basic ideas and concepts. If you can find an answer that allows you to install what you are looking for, I would very much like to know about it, as many people here ask the same question you are asking.
However, (and I hate to sound redundant) the blur-less ROMS are created because many people dislike blur. Along with that... If I can find an answer to help you out, I will be sure to post back here as well!
Sorry to sound repetitive or a downer, just trying to provide you with any info I have.