Oil coloration

Could be something as simple as condensation, or as serious as a cracked block. Drain the oil and change the filter, then refill and drive it for a couple of miles at an easy pace. Let it idle and check the outside of the block carefully for obvious fluid leakage from a cracked block on the outside. If none, do another oil and filter change. Check yoor exhaust color to see for a cracked head or block allowing coolant into the combustion area, and do a compression check, too. If you still don't see any problems, and assuming the crack is not on the inside of the block, you've likely got a simple condensation problem. That's about the only thing I can tell you. Others may have their own methods of checking for this problem.