I know antique black powder guns, and replica's there of are not required to be locked up. Not sure about "modern" type of black powder guns (thompson center for example).
trigger locks would have really taken away from my 1853 enfield, and 1863 springfield repros hanging over the fireplace.
This is in no way legal advice......local PD may have differing opinions....that's what you have to worry about.
Black Powder guns in any configuration (except the TC Encore type that will accept modern rifle cartridges) are not considered firearms per say. You can buy them thru the mail and need no FID to own them. Therefore I beleive it legal to have no trigger lock on them or they be stored in a safe. Obviously the percussion cap or 209 primer should be removed...or better the load discharged. I beleive there is more confusion where the game warden may consider the gun unloaded when no cap or primer is in place. But Local law enforcement may only consider it unloaded when the load is discharged.
The funny thing is I beleive black powder requires an FID to possess....
There's grey area all over the place when it comes to Black Powder guns....