1 The Hub for example isn't stale yet and is still receiving improvements. 10.3 brings Instant Actions (Ref:
Thank You BlackBerry for making email triage on 10.3 faster than iOS and Android | CrackBerry.com ), previous updates brought the priority Hub, pinch to sort, and even a gesture to quickly move to the next/previous email. These are pretty major things to some people, and you really can't expect a complete over hall to what is typically called the phones best feature. Things will come, it way seem slow, but proper development and testing takes time. Toast notifications, not really sure what else you would want...
2. So, back to Blend... I see two major things here, first as I already mentioned there is the possibility it will eventually move to be accessible through any browser. There were rumors this was originally the plan, and rumors that the software is nothing more then a browser. Second, it appears Blend has the framework to work with any app. Assuming BB releases an API for it developers should be able to integrate their own apps into Blend. Look at BBM in Blend, it has it's own so called app at the bottom, where you can access channels, but it is also accessible through the Blend Hub.
3. Lets assume BB does release an API, which I think it is safe to assume, it is then up to the developers to make/update apps to use that API. That will be dependent on its overall popularity. Just keep in mind, the API is the big thing, this is what will allow Blend to do amazing things... it really doesn't need to go much further then that. BB has already delivered a very complete solution that works very well.