Add borders to SVG path d3 javascript

I want to have my svg paths (lines) have borders such that outlines are black, but the inside is another color. Line code:

self.lineFunction = function(dat) {
  var self = this
  var line = d3.svg.line().interpolate('linear');

  var data = dat.map(function(d) {
    return [self.xScale(d[0]), self.yScale(d[1].mean)];
  });

  return line(data);
}

self.lines = self.svg.selectAll('.line')
  .data(d3.keys(self.data), function(d) {return d})
  .enter()
  .append('path')
  .attr('d', function(d) {return self.lineFunction(self.data[d])})
  .attr('class', 'line')
  .style('stroke', 'blue')
  .style('stroke-width', '2')
  .style('fill', 'none');