Important!
(This fix is only for the original Blood)
A lot of people have been reporting flicker and slowdowns when running Blood in resolutions higher than 800x600.
To all of you guys, there is a pretty simple way around this problem.
In your Blood directory go find the file (dosboxBlood.conf) and open it using notepad, find the line (fullresolution=original) and simply replace "Original" with your choice of resolution.
Example "fullresolution=1152x864" without the quotes.
Save the changes and go back and run the game, DosBox will now upscale Blood to the resolution you have just specified.
Important!
By making this change to the (dosboxBlood.conf) DosBox will upscale Blood to the resolution that you have specified, and performance may vary depending on your computers ability to run DosBox.
Enjoy Blood.

Well Rene, there's no doxboxBlood.conf in my blood2 folder (or subfolder).
I always thought that Blood2 is a windows game.

This fix is for Blood 1, not Blood 2: The Chosen.
Blood 2 is not using DosBox.

This doesn't increase the game's resolution.
It changes the resolution of the monitor, and filters the pixels.
Kind of like dvd up-sampling.

No matter what it does, it gave me one hell of a graphical upgrade!

This is why you need to set the in-game resolution via the file blood.cfg as well (the strings ScreenWidth and ScreenHeight). The first poster presumes you know how to do this in his instructions. For me personally, even with these instructions, 1024x768 still flickers, but 1280x1024 doesn't.

Yeah, in game 1280x1024 doesn't flicker on me either, but it's near unplayably laggy.
Actually I prefer to play in 640x480 for the smooth framerate, though 800x600 is not bad either.

Works wonderful, especially with the launcher :)

I don't know if he really was presuming that or not, but it wasn't mentioned, or how it was worded.
1024x768 flickering may be a dosbox issue, which if is the case, cannot be fixed without a version update.

I don't think there ever will be a fix unless the source code is released.