If you did this, wouldn't users just remove the blocking task relationship so that they could close the task?

It's ok if they do that. The system needs to be right. If users find ways to cheat the system that is fine by me :-)

@chad I am sorry, can you mention these ways around in detail. I am not sure if I caught one. Don't you think blocking tasks should inherently block the parent tasks else what is the difference between blocking tasks and subtasks?

Sorry if I am rude but every time I come on this forum I get no help whatsoever.

I feel you should trust your developers, they are hopefully smart, well vetted and well managed. We don't want Phabricator to get in the way of a productive employee, so we try not to add arbitrary blocks unless there is a real problem that it solves (like say lint, unit test). The way around was mentioned above by @joshuaspence, so I assumed you had read it. A user can remove the relationship and still close the task. We have many 'trust' features in Phabricator and I consider it a core part of our product. I trust anyone to edit a task, anyone to change relationships, anyone to manage their workboard, and anyone to close a task.

We generally prefer feature requests just state problems, as in what real problem did you have using Phabricator. It can be a confusing way to think about a request, but it's very helpful for us since we don't have insight into what you're actually trying to accomplish. With requests that are just "add this feature because it doesn't exist" we have a very hard time guessing on how we can help you with your problem. The real scenarios are what helps us most.

https://secure.phabricator.com/book/phabcontrib/article/feature_requests/