On a shot where you your colored balls are still on the table, you cannot call the eight ball.
3.6 Shots Required to Be Called
The eight ball may be called only after the shot on which the shooter’s group has been cleared from the table.
So on the shot you've described, the eight ball could not have been legally called. In that case, sinking the ball is a loss.
3.8 Losing the Rack
The shooter loses if he
(a) pockets the eight ball and fouls.;
(b) pockets the eight ball before his group is cleared;
(c) pockets the
eight ball in an uncalled pocket; or
(d) drives the eight ball off the