1) DosBox is a memory hog. The problem might simply be that your computer isn't good enough to run said game through DosBox. The best solution would be to just try running it WITHOUT DosBox, and cross your fingers that it will work.
2) Certain game simply don't play well with modern computers. Build engine FPSs are a good example of this. Even when running through DosBox, games like Blood can run into issues. This might be the case with your game. I don't know for sure.
3) There isn't a good alternative to DosBox, short of finding a fan-made windows port (ex. WinRott for Rise of the Triad) or booting up an old machine that can access Dos. Don't blame GOG for using it, because it's probably the single best way to get Dos-based games running on newer systems. But like anything, it'd not perfect.
4) My first two purchases from GOG were Blood and Rise of the Triad, neither of which worked from the start. This is by no means representative of the GOG catalog as a whole, since most titles on here have ended up working fine for me. Have some patience. These are titles that were never meant to work on newer systems, so OF COURSE there are going to be some problems. In some cases, the fact that GOG got the games running at all is a feat in itself.
jamyskis: Honestly, if we're looking to encourage people to buy at GOG, people need to stop behaving like fucking religious zealots and start helping.
wait...you want people on the internet to act like adults???
MADNESS!!!