Have you published the page the Blog Application is attached to?@TheRocket786: I don't think this is the case :).
Even if the page hasn't been published yet, it does have an ID already.
The exception as the one above can be thrown if you, for example, try and add a blog entry directly in the Data perspective to "Composite.Community.Blog.Entries".
In this case, the manadatory field "PageId" can't be set at all, hence the exception.
And this field is indeed hidden in the data form, because a dynamic form is used for adding and editing items in this case.
Hope this info can help @c2w