I see....I fingered it out. It blends on the surface and I was trying to blend them on the screen. Here is a sample.
surface_img:draw(screen)
--fill suface with a orange color
surface_img:fill_color({245,68,0})
--"blend" blend mode.
blend_test_img:draw(screen)
blend_red_img:draw(surface_img,100,200)
blend_red_img:set_blend_mode("blend")
blend_green_img:draw(blend_test_img,100,200)
blend_green_img:set_blend_mode("blend")
blend_blue_img:draw(blend_test_img,100,200)
blend_blue_img:set_blend_mode("blend")
surface_img:set_opacity(80)