I've got a few video converter boxes (Marshall VAC-11SU3, Marshall VAC-11HU3, Magewell USB Capture SDI, Blackmagic UltraStudio Express) and no cameras. They all have an incoming video signal plugged into their respective SDI or HDMI ports.
The issue is that GetNativeMediaType always returns the same format as GetMediaTypeByIndex does for index 0 regardless of the actual video format that is coming into the SDI/HDMI port.
Every Media Foundation example I've seen so far has a UI to pick the "correct" native format. This menu is populated from GetMediaTypeCount and GetMediaTypeByIndex for the device.
My users will not know what to pick!
We've been using Blackmagic's DeckLink APIs and our users see the incoming video signal format in the UI. We'd like to expand support for multiple device manufacturers but this one has me stumped.
Media Foundation does not employ a concept of signal format detection you have with recent Blackmagic hardware (earlier Blackmagic products, by the way, did not offer detection).
A video source driver could indeed enumerate the media type it sees on the wire as first GetNativeMediaType output and/or offer dynamic format change during streaming session to such format. Media Foundation video sources are mostly assuming however webcamera-like devices and have a fixed type enumeration order.
I would not assume Blackmagic driver to be different because it mostly mimics a webcamera, so that with a WDM driver Blackmagic device inputs could be consumed using standard APIs. If one needs extended functionality, such as signal detection, Blackmagic suggests using their DeckLink SDK (which is good by the way).