The black mark does not look like any pest-related or pathogen-caused damage that we are familiar with and it's probably not of concern. The yellowing leaves are dying because the plant is cutting them off from their moisture and nutrient supply (just like trees losing leaves in autumn do), and as you correctly point out, this is perfectly normal as leaves age while plants are in active growth, so it's not uncommon for opportunistic decay organisms like harmless fungi to colonize these shedding leaves and begin to cause spotting or dark areas, which might be what this is. Otherwise, maybe it's just the tissues degrading as the leaf dies off. No treatment is necessary, but if concerning symptoms appear on leaves still alive, then feel free to send us additional photos for diagnosis. (At that point it would help to know what kind of Hibiscus this is -- winter-hardy or tropical -- and whether it's being grown in a container or in the ground. For this issue, though, that doesn't matter much.)

Miri