Single Source of Truth
One important principle in program design is having a single source of truth for a specific program state. So if you have several ways of determining if a Page is a Blog page, this can get confusing and result in bugs.
Instead, you should have just one place where the Page type is stored, and access it in a uniform way through the program. This principle is called the "single source of truth" and is well-known in computer science. It can be applied to many situations, including:
Programs (like with the Page example).
Databases, where only one table returns the result (rather than many tables).
More complex systems with multiple programs and databases.
As always, using caches to speed up access to this information can be problematic, as the cache ends up copying the single source of truth. Even when writing a simple program, relying on a unified, single source of truth is helpful.