How to restrain the arbor on a circular saw to change the blade

Honestly, a 12 year old black and decker probably needs to go to the scrap yard. Low end saws are not built to be rebuildable, at least since the late 20th century (some 50 year old "low end" saws were a lot less "low end" in construction) - a 21st century low-end saw was usually built with "planned obsolescence" from the design phase forward.

Provide the exact model of your saw to check some details.

The one thing (if that's been left serviceable on this model) it might be worth trying is to replace the brushes if they are excessively worn. Beyond that, replacing the saw is far more likely to be cost-effective.

It is very likely, that the fixating screw/disk is loose. Do not operate the saw in this state. It is dangerous.

A circular saw usually has a direct drive. I.e. the motor spindle is also the center shaft for the blade. So if your motor is spinning, but not the blade, it means the blade has lost contact to the shaft.

Check if the blade is properly centered and aligned with all locating pins (if your saw has some). Then fixate the screw using the appropriate wrenches.