For a 1995, given that it's 26 years old (big emphasis on age hence the BOLD) and 188K, you're "in the zone" for head gasket replacement.
You wrote earlier that this overheating condition gradually started to show up the past couple of summers...
To me, this is the initial signature of a failing head gasket before actually blowing it out .
That small black stuff you're seeing in your coolant might be exhaust, oil, or head gasket particles, or a combination of all.
Long life coolants have made this condition not as frequently seen compared to when we were growing up with only choice being "green" coolant, BUT head gaskets haven't changed and they do have a service life.
Think about it, yours have had 26 years of daily heat cycles and they might be telling you something.