There are a few ways to do this. The first is with colour/lighting. You can achieve this by using Adjustment Layers.
Start with a Gradient Map with the the first colour as your main colour in your sig. The end colour should be black. Set this Layer on Hue and set it to about 50% Opacity. This will keep the colouring consistent throughout the sig. Then I usuall make an Exposure Layer, with the settings Exposure: 0, Offset: +0.0165, Gamma: 0.9.
Then make a layer and use a large, round, soft brush with black at about 40% opacity (the brush not the layer) and brush the background. Then use white and brush around the render. Set this layer on Soft Light.
Next way to blend is smudging. There is no right way to do it, but I'll give you some tips. This is my recent Boychuk sig before any stocks.
It looks like a part of the sig before adding anything else. Use a brush like this:
Make the size between 10px-20px. Strength: 50-60%. Duplicate your render layer and start smudging the edges in and out. Make the original layer invisible to see how it looks. Repeat these steps as necessary.
Finally, you use stocks. You already seem to be getting this, but a few tips. Put the stocks on top of the render, and if necessary erase some parts over the render. If it's too bright when it's set on Screen or Lighten, try pressing Cmd+L when the layer is selected, and move around the middle slider until it looks better.