The blue eyeshadows make your brown eyes stand out, while blue shadows make blue eyes seem almost bluer. Brown shadows tend to frame blue eyes, while complimenting brown eyes.
Whether you wear these two colors together or apart on your lids, you should see what they do for your blue or brown eyes.
One good tip regarding blue shadows is that they should always be a little darker than your natural blue eye color, and not very light or the same shade of blue as your own eyes.
If you have brown eyes, you can wear almost any shade of blue eyeshadow, although you should try to get the shade to complement your skin tone as well as your eyes. Lighter shades of brown look best on blue-eyed people while most shades of brown work on brown-eyed people. Keep these things in mind when choosing blue and/or brown shades of eyeshadow.
If you want to wear blue and brown shadows together, try to keep warm shades with warm shades and cool shades with cool shades. There are warmer blues, and these look very nice with warmer browns. The cooler, darker brown shades of eyeshadow look very nice with navies, royal blues, and vibrant blues.
If it helps, think chocolate brown with royal blue and light coffee with powder blue when you are trying to decide how to mix and match your blues and browns during shopping and application processes.
Your blues should almost always be applied on the lower lids or at the corners as an accent color after applying your shade of brown eyeshadow.