Here is the full list of all 50 Black Mesa achievements. It takes around 20-25 hours to unlock all of the achievements on Windows.
Carry the Hidden Hat to Endgame.
Decline G-Man’s offer of employment.
View the *other* secret ending.
Defeat Nihilanth in under 4 minutes.
Take out all three Healing Crystals in Nihilanth’s Chamber at the same time.
Accept G-Man’s offer of employment.
Don't kill any Vortigaunts in the Borderworld.
Kill a Controller without killing any of its mind-controlled Vortigaunt slaves.
Kill a Headcrab using fire.
Get overtaken by a Headcrab's ample anterior.
Drown in Lambda Core's coolant.
Kill 10 enemies with Snarks.
Send the Hidden Hat to Xen.
Wait too long to jump into the Lambda Teleporter.
Overcharge it anyway.
Use an entire clip of Gluon ammo in one continuous shot.
Kill a Barnacle with a Satchel.
Kill 10 Airborne Headcrabs using the Shotgun.
Escape the Hangar in Surface Tension without alerting the Soldiers.
Kill 20 enemies with the Hivehand.
Kill 10 enemies with a reprogrammed Sentry.
Get killed by your own Snarks.
Kill 5 enemies with one grenade.
Cook a Grenade for too long.
Perform surgical caffeine extraction from 20 machines.
Kill an Assassin with the .357 Magnum.
Kill 10 Enemies with Tripmines.
Headshot 20 enemies with the .357 magnum.
Kill a Marine with their own grenade.
Assemble all the possible Scientists and Guards at the end of Office Complex.
Pass the toilet paper to the scientist in need.
Ruin the microwave casserole.
Conduct all of the questionable research experiments.
Kill an Icthyosaur with either the Tau or Gluon.
Use the alternate path in Office Complex.
Interfere with the alarm and laptop in Anomalous Materials.
Refuse to insert the specimen into the Anti-Mass Spectrometer.
Help the Barnacle and Headcrab find true love.
Kill all Protozoans in the Borderworld.
Die in the AMS Beam before starting the experiment.
Skip the opening Xen island with a secret jump.
Use the welcome mat.
Deliver the finishing blow to Gonarch with your Crowbar.
Get whacked into oblivion in the ledge fight.
Find each member of the missing research teams.
Use Cyanogen to kill Gonarch.
Don't use the Cyanogen and let the Gonarch live.
Find the snack supply canister.
Complete Interloper without taking any laser damage.
Escape the Gargantua’s Lair without taking any damage from Gargantuas.