Weathing powders are amazing. I love them. They allow you add matt colour to your model rather than the glossy finish of washes. I use a simple dark brown taken straight from the pot and lightly brushed over the metal parts. Its OK if you get some on the red because its just further dirt. I then re highlight the metal using mithril silver but only fine lines of it.
As far as the blood goes, so far I've always mixed it by trial and error out of perhaps 2 parts black wash, 2 parts red wash and 1 part red gore paint. The result is a fairly transparent gore but with plenty of dark patches where its thicker. I finally paint drops of pure black wash on the "thicker" patches of gore, where more blood collects.
To do the strings of blood, simply dip a pair of cocktail sticks into UHU glue in the yellow tube. Its really stringy and can be mixed with your blood paint to make it like gore.