If you mean the font it uses when no font is loaded, just free whatever you're using:
If you mean blitz.fon, I don't think that Blitz can load anything but TrueType fonts. Blitz.fon looks to be what the IDE uses (and since it's a bitmapped font, it doesn't scale well as you can see by double-clicking on it, so you probably wouldn't want to use it anyway).
Open in online IDEGraphics 800,600,16 Big=LoadFont ("Arial",40) SetFont big Text 10,10,"foo" FreeFont big Text 100,100,"bar" WaitKey
If you mean blitz.fon, I don't think that Blitz can load anything but TrueType fonts. Blitz.fon looks to be what the IDE uses (and since it's a bitmapped font, it doesn't scale well as you can see by double-clicking on it, so you probably wouldn't want to use it anyway).