By the looks of things, we are into "B" syrup season. Maple syrup is graded on its quality and color; ranging from lightest amber to dark, rich brown. (That's actually density and translucency if you want to get technical) While the tourists seem to gravitate towards the lighter stuff, those of us diehard maple lovers hoard the darker, "B" syrup. It's where the real maple flavor is at...
Can't say as I know all the reasons why late season sap is darker, but it may have something to do with cooking time...it took almost the same amount of sap to produce both jars, so there was more water in the later stuff to be boiled off. Longer boil = longer time = darker color (think of carmelizing sugar when cooking).
There's another quart on the stove finishing right now, so we'll see how permanent this darker phase may be. The forecaster is calling for a foot of snow tomorrow - someone better tell the maples it ain't over yet!