Sheesh Buckom, don't go messin' around with my Dark Brothers and Sisters!!!!!!!!!! LOL
I think the OP is more looking for "real' consequences. Or maybe I am. For example, if I join the Dark Brotherhood, all the other factions are ok with it. Or, I'm part of the Thieves Guild and even so I shake down the merchants in Riften, they still trade with me. They might grumble a bit, but really, there is no real disposition check in Skyrim. Nothing you do really affects the way you are treated.
Considering you're blackmailing the merchants, they no better than to mess with you. Especially after Loud and Clear.
As for the DB questline and people being okay with it, I've always did everything I could to make sure nobody could actually know I was in the DB. I wore the outfit on jobs, but traveling to or from them I either A) Dressed in civilian garb,

Traveled through the wild and only at night so as to avoid coming across anyone, or C) Used free Illusion spells to spam Invisible.