It depends on the definition of "blue collar" I guess.
True, imported labor tends to initially be used in the agriculture and service industries, then migrates into better paying sectors of the work force.
What will be the saturation point is anybody's guess.
I work in a large machine shop where communication is important. We'd have a hard time building anything if we had people who did not speak english working there.
We're well past the saturation point ---- but welfare and all the many government poverty programs are covering that up ---- what happens when you try to cut back on any of these programs? When the food stamp allotment is cut back or free health care is denied? If the welfare class ever had to go out and try and get a job to feed themselves and found there are no jobs at all for them to do.