Got it now. If the SEL ALT is on the opposite side of where the A/C is going then the ALT(blue) is not displayed. That is logical since the ALT is not armed for obvious reasons.
So I was wrong. If the SEL ALT is moved to one direction, and A/C sent the other way, the ALT(blue) won't be there. I still have a bit brainwork to do in order to build an image how that would happen and be left unnoticed via the other markers. Yet if there's a need to shield your self from that, reading ALT(blue) is a sensible thing to do I guess.
Stay safe out there. And thanks for the effort of opening my skull.