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.
Kill all Protozoans in the Borderworld.
Find each member of the missing research teams.
Escape the Gargantua’s Lair without taking any damage from Gargantuas.
View the *other* secret ending.
Defeat Nihilanth in under 4 minutes.
Carry the Hidden Hat to Endgame.
Decline G-Man’s offer of employment.
Accept 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.
Complete Interloper without taking any laser damage.
Find the snack supply canister.
Kill a Controller without killing any of its mind-controlled Vortigaunt slaves.
Deliver the finishing blow to Gonarch with your Crowbar.
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.
Kill an Icthyosaur with either the Tau or Gluon.
Skip the opening Xen island with a secret jump.
Send the Hidden Hat to Xen.
Wait too long to jump into the Lambda Teleporter.
Drown in Lambda Core's coolant.
Escape the Hangar in Surface Tension without alerting the Soldiers.
Kill 10 enemies with Snarks.
Overcharge it anyway.
Conduct all of the questionable research experiments.
Get killed by your own Snarks.
Get overtaken by a Headcrab's ample anterior.
Kill 10 Enemies with Tripmines.
Help the Barnacle and Headcrab find true love.
Kill a Marine with their own grenade.
Kill 10 enemies with a reprogrammed Sentry.
Use the alternate path in Office Complex.
Assemble all the possible Scientists and Guards at the end of Office Complex.
Kill 5 enemies with one grenade.
Refuse to insert the specimen into the Anti-Mass Spectrometer.
Kill 20 enemies with the Hivehand.
Kill a Barnacle with a Satchel.
Kill a Headcrab using fire.
Use an entire clip of Gluon ammo in one continuous shot.
Kill an Assassin with the .357 Magnum.
Headshot 20 enemies with the .357 magnum.
Kill 10 Airborne Headcrabs using the Shotgun.
Cook a Grenade for too long.
Perform surgical caffeine extraction from 20 machines.
Die in the AMS Beam before starting the experiment.
Pass the toilet paper to the scientist in need.
Ruin the microwave casserole.
Interfere with the alarm and laptop in Anomalous Materials.