Blood actually uses an advanced version of the Duke Nukem Build raycaster Engine. The level editor back at that time was more or less revolutionary (imho!), very well documented and very good to handle. It felt more like they ran a bit out of ideas, as the quality of the levels in Blood differs significantly. At some points I had the impression the developers tried to expand the content after finding out they're running out of time. So build some annoying areas and throw mobs inside.