Those aren't blu-ray videos, my friend. They're what are called Matroska video files. (Matroska format is what's called a container format, allowing various codecs to be incorporated. Codecs are files used in the COmpression and DECompression of audio and video files. Did you notice my use of capital letters in that last sentence? That's where the acronym codec originates from.)
Anyway, that's beside the point. You were asking how to play .mkv files. The easiest way is to download and install VideoLan VLC Media Player. This FREE programme comes with it's own built-in codecs, and can play almost every file that you throw at it.