Fiz assim
canvas width="600" height="400"></canvas>
<script>
var tela = document.querySelector('canvas');
var pincel = tela.getContext('2d');
pincel.fillStyle = 'black';
pincel.beginPath();
pincel.moveTo(50, 50);
pincel.lineTo(50, 400);
pincel.lineTo(400, 400);
pincel.fill();
pincel.fillStyle = 'white';
pincel.beginPath();
pincel.moveTo(100,175);
pincel.lineTo(100, 350);
pincel.lineTo(275, 350);
pincel.fill();
</script>