We have the Japanese beetles here as well and I don't know of anything that kills them without hurting the blooms. With roses I just cut them back by a foot and remove every rose so they have nothing on which to feast and I guess all you could do is cut off the dahlias, try your best to knock off and kill the beetles, and keep the dahlias outside in case there's one hidden in between the petals.
I spray with Spinosad, don't know if it helps with those monsters, but it kills a lot of chewing pests, and its organic. The saving grace is that the little jerks are mostly around for only 3 weeks, as opposed to longer term pests. If they are local, using milky spore or watering in nematodes in the ground before they emerge from the soil can get rid of them. That has to be done in Spring though. If you have an area far away from your valuable plants, a bait trap can divert them and kill them. But don't put it around your plants as it will be JB central.