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