Clear Red is also more like fresh blood. For older blood I try to mix it with clear green (another tamiya color) to achieve a slightly brownish color. Both colors stay glossy/shiny.
well death guard is just all rotten, guess every wound will be extremly infected :) Pus everywhere! A little bit of purple like @evdoggformayor suggested fits in there . Nurgling Green sounds interessting, but I'd still use some yellow tone along the way as you said. At the end of the day it depends on the color one ownes. Still have the old Camo Green (equivalent to the current Elysian Green if you believe the conversion charts) laying around, guess this kind of color could be used, too.