[ammojamo]

Disagree. The purpose of the indicator is to signal your intention to others. Indicate first, then shoulder check. If you then discover a car in your blind spot, abort the manoeuvre if necessary or slow down to let them pass you first.

I do agree it's bad form to indicate when you know there is a car in your blind spot already.

Discovering a car in your blind spot should be a relatively rare occurrence if you are driving attentively.