If you carefully look at the latest video Valve uploaded on their YouTube channel, the delivery of a Portal 2 Sentry in their HQ, you will notice, at 1:11, a guy wearing a Black Mesa grey sweater!
https://www.youtube.com/watch?v=LvZhRUsKNG4
Let the conspiracy theories begin. 
Valve made up the Black Mesa Research Facility
It’s a half-life sweater
Perhaps It’s not refering to the mod but just the research facility, a reference to the mod would be awesome though.
Oh wow, a Valve employee is a wearing a shirt referring a place from one of their games? Stop the presses!
Aaaaaaaaaaah, dammit… just checked on the Valve Store… I thought they weren’t selling that exact sweater, but… https://store.valvesoftware.com/product.php?i=C0112
[all excitement disappears]
…but it would have been cool if there was some connection with the mod. 
The BM Team doesn’t sell any merchandise related to the mod, so the only logical answer is that it is an official Valve sweater.
Excitement got over logic for a moment. 
Sorry for hyping for nothing!
Mods, you shall close this.
Yeah well, nice try.
Founded in 2004, Leakfree.org became one of the first online communities dedicated to Valve’s Source engine development. It is more famously known for the formation of Black Mesa: Source under the 'Leakfree Modification Team' handle in September 2004.