It sounds like you have already identified the blue boundary somehow, because your question does not ask how to get the boundary in the first place. So, having the boundary, you should have an ordered list of x,y coordinates for it. So then, to create a mask that is 1 inside of the boundary and 0 outside the boundary, use poly2mask():
[rows, columns, numberOfColorChannels] = size(grayImage);
mask = poly2mask(x, y, rows, columns);
