Trail Boss is a modern nitro (smokeless) propellant. Its burn characteristics are just like any other smokeless pistol powder. It was developed for the cowboy action shooting market. Many cowboy action shooters use old cartridges like the .45 Colt and .44-40 that were originally designed for black powder. As a result, the case capacity is much larger than it needs to be if you are loading nitro propellants. That can lead to several problems. The biggest problem with nitro propellant in those big old black powder cases is that you run an elevated risk of double charges. Because the correct charge of a nitro propellant like HP-38 takes up only a small fraction of the case capacity, you can easily double or even triple charge the case and the charge will not overflow. Indeed, it may still be so far down in the case that you won't notice it without shining a light down into the case and checking specifically for an overcharge. As you might imagine, pulling the trigger on a Colt SAA with a double or triple charge of nitro propellant in the cartridge under the hammer can result in a pretty impressive KA-BOOM. Say bye-bye to your gun, and perhaps parts of your hand, eyes, etc.
Trail Boss mitigates that risk because it takes up a much larger volume than conventional nitro propellants. With Trail Boss, if you double charge evan a large pistol case like .45 Colt, the charge will overflow the case -- a pretty positive alert that you've screwed up and need to start over.
The other issue with nitro propellants in big cases is powder position sensitivity. It is well documented that you can get extreme variations in velocity depending on where the powder is in the case when it ignites. That's not an issue if the powder fills the case, because then it can't move around. If you have a small charge of nitro powder in a big case like .45 Colt, though, there is a lot of air space left over. That powder may be sitting back against the primer at ignition, or up against the bullet, or it may be distributed along the wall of the case. People have published data showing the big differences generated by something as simple as pointing the gun up before firing (so the powder is back the primer), versus pointing is down before firing (so the powder is against the bullet).
With Trail Boss, the powder takes up so much space that there is less potential for variability due to powder position in the case. That should theoretically lead to lower ES and SD, and better accuracy.
In general, Trail Boss is perfectly acceptable for black powder cartridge arms, just as any pistol or shotgun powder is generally acceptable for them. While I mostly load black powder for my .45 Colt, .44 Russian and .44-40 guns, I do load some smokeless rounds for practice and for friends who want to shoot them without the smoke. I use HP-38 and Hodgdon's Clays, but IMR Trail Boss would work fine, too. Just stick with published load data and you'll have no problems.