You'll probably see San Blas Cathedral across town before you actually find this important religious building. Peek inside if the doors are open or just admire the facade from the street.
If you're spending a while in Barrio Centro, San Blas Cathedral is a good place to start your exploring. This principal church is one of many attractions that'll keep you on the go here. Friendship Bridge, Triple Frontier, and Three Borders Mark are also waiting to be discovered nearby.