I use friXion pens but mine are all scrappy so if there is a faint white line on the darker fabs- it doesnt bother me.. and unless anyone is looking for it no can tell.. otherwise ill use a regular mechanical lead pencil to mark the wrong side..
I hate the blue pens, they seem to dry up and stretch the fbs when im marking..
Ill use Fons and Porter White marking Meachaincal pencil on darker fabs - it washes out great..