You didn't say if you were using DexCool (orange stuff) or polyethylene glycol (green stuff) anti-freeze. DexCool turns brown rather easily. The green stuff can look like pond scum after a while.
You also didn't specifically say if you had changed out the freeze plugs. If they are getting a little weepy, now may be a good time to change them out. You'll need to drain the block. While the freeze plugs are out you will have access to the bottom of the cylinders where a lot of rust settles. You can either let it set a few days or use a hair dryer to get rid of the moisture. Once dry, rust clumps can be removed through the freeze plugs with a magnet-on-a-stick kind of thing. You'll be amazed.
Getting the big crud out will make it easier to get the little crud out. And getting the crud out is what flushing the block is about. Besides clogging up the radiator and heater, it can also compromise even heating/cooling of the block.
Unless I'm dealing with substantial amount of aluminum in the cooling system, I recommend a 50/50 mix of the green stuff. When I mix my own, I use distilled water in order to keep excess minerals, especially calcium, out of the cooling system. You can get distilled water at the grocery store. Because I'm old school, I also dump in a can of water pump lubricant when I change the coolant. Don't know if I need to do it, but I've never had a water pump failure.