I am looking into making a cross platform mobile application. I know that the apple store won't allow you to download and install an app that is over 50MB. Do the google or blackberry app stores have a similar restriction. I can't find anything that doesn't say they don't, but I don't want to assume.
If you're referring to the legacy BlackBerry Java platform, the maximum size of an app is actually only about 14 MB.
If you're referring to the new BlackBerry OS's, see this document:
BlackBerry 10 and BlackBerry PlayBook OS version 1.0.6 and later supports applications (BAR files) up to 2 GB in size.
BlackBerry PlayBook OS version 1.0.5 and earlier supports applications (BAR files) up to 300 MB in size.
On Android, the size is for all intents and purposes unlimited (50MB + 2GB + 2GB), if you use expansion files. Of course, if you're on a 3G connection, it's going to be quite slow to download apps that are larger than the old 50MB hard maximum, and may incur significant costs for your users with data limited plans.
For completeness, here's the Apple link stating that the current Over-the-Air download limit is 50MB, as you said.
Note: the Google/BlackBerry sizes are limits that do not depend on the download connection type (e.g. carrier network vs. Wi-Fi). Your users may have additional limits imposed by their carriers, but that's impossible to comment on without knowing which carriers you're targeting.