That is because the default particle is of a circular gradient, which means the center has an opacity of 100%, but the edges have an opacity of 0. The easiest solution is to do this:
void Start () {
ParticleSystem renderer = this.GetComponent<ParticleSystem> ();
renderer.startColor = Color.blue;
}
Or
1. Create new Material
2. Change the shader to ‘Particles / Alpha Blend’ then change the tint color to whatever you like
3. Then assign this material to the ‘Material’ component inside of the ‘Renderer’ tab.
Hope this helps!
TBART82