The black dot tells you the phasing between the windings; which end you
need to hook where in order to avoid a 180 degree phase shift. If you
look at the primary and secondary voltages on the dotted end of each
winding with respect to the undotted end, they'll be in phase.

jawod wrote:
On schematics, there can be a black dot on one end of inductor symbol in
a transformer. What does it mean?

I know, pretty basic, huh?