Hello and Welcome!
Well, from the place you loose the blood trail you can continue in a circle patern, expanding the radious few feet at the time depending on the terain. Other method is to follow a grid patern and search every square from the grid in systemathic way...sound complicated but is easy...or you can buy one of these
http://www.cabelas.com/cabelas/en/templates/links/link.jsp?id=0015690224984a&type=product&cm Cat=search&returnPage=search-results1.jsp&QueryText=blood+trail&N=4887& amp;Ntk=Products&Ntx=mode+matchall&Nty=1&a mp;Ntt=blood+trail&noImage=0
Also found this...intersting..
"Here is one way to find a faint blood trail when tracking a wounded deer. Pour about eight ounces of hydrogen peroxide into a spray bottle. Add some yellow food coloring to it. Go to the last visible blood sign and lightly spray the surrounding area, grass, leaves, and bushes. The peroxide foams up when it contacts the smallest drop of blood The yellow food coloring makes it more visible."
good luck...