I have an old truck that I inherited from my grandfather after he passed. It has 483k miles on it, and had been sitting for over 2 years before I got ahold of it. When I drained the old oil out, it was jet black and had the consistency of pancake batter. I put in PYB 10w-30 and let it idle for 15 minutes. Then I drained it and it came out black already. I put PYB back in and haven't messed with it since. I'm not sure if I should add a quart of MMO, let it run another 15 minutes, and dump again... or if there's a better oil for cleaning out such a situation.
First off, don't worry about the oil being jet black. I'm fully aware that it worries folks in the US when it gets that bad but to me, an oil can be jet black and still be fully functional. Diesel oils can happily do HUGE extended drains and spend their entire life black.
Second, don't worry about the 'pancake batter' appearance. It doesn't mean the engine has sludged. At a guess, there was a bit of dispersant VII in the old oil. It tends to impart a odd 'gloopiness' quality to the oil which can be a bit disconcerting the first time you see it.
Third, be aware that when you do drain an oil, a lot of it can remain held up in the block. I once did an engine test (it might even have been a clean-up test) where I ran a full Ca-based oil, drained it & then added a full Mg-based oil. I was rather surprised afterwards to find so much Ca showing up in the newly added oil and back calculated that 20% of the original oil hadn't drained. I suspect the black 10W30 you drained was black because it intermingled with old undrained oil (not sludge). Had you done a second & third oil change, it probably would have eventually drained clear(ish) but it's a bit of a waste of new oil.
One question? Have you taken off the rocker cover yet, or better still, unbolted the oil pan? What do you see? You will be able to spot proper sludge straightaway. 483k is a lot of miles and you may well have sludge but if the oil's been changed regularly throughout it's life, it could still be good.