Not sure if the pictures are helping me. In some angles, it looks dark below the tree and from other angles, it does not look so dark. You may need to raise the tree canopy to let more sun inside.
Hydrangeas do not need more than a few hours of sun down here or dappled sun. They may thus do fine if the light underneath is not dense shade. One my my sister's neighbor's has a mophead in less than full bright shade and it always bloomed fine. However, they may require winter protection to bloom reliably if the ones selected only bloom on old wood. In cold winters, the early Spring blooms may get zapped and you will have to wait for the new wood to grow larger/older, develop flower buds and open blooms... so you may get blooms in mid to late summer. TAS is a copy of Hydrangea Pistachio, which performs well for me here. An acquaintance who has a Bloomstruck is very happy with the ton of blooms that he got this year but I doubt one could generalize from just one person's experience inn one year. ;o) It is encouraging though. Of the two, Pistachio/TAS seem more compact to me. Not sure how the temps are down there in MA now but you may want to delay planting for later if it is getting hot.
I an unaware of hydrangeas causing issues to trees. Instead, the issue seems to be when a tree has a lot of roots in the top few inches and when these compete with the hydrangea for water and minerals. As long as the area does not show jillions of roots where you dig to place a hydrangea, it should be fine. I am not aware of oaks being black listed by hydrangeas. You could test the waters with a few shrubs at first and then add more if all goes ok.
