mercruiser doesn't "BUILD" much other than outdrives. There are some Merc-specific parts out there, such as thermostat housings and such, but most Merc-stamped parts are just rebranded stuff.
Not saying Merc won't or can't build whatever they want, but it is usually more cost effective to use an existing part. Might need a tweak or two for Merc usage, but usually not even.
To my knowledge, though, Merc has never attempted to make their own blower.
Weiand and B&M have been used on their smaller SC motors.