Blend tool is used to blend two overlapping objects, but this tool depends on two variables: the RGB color of object and the opacity. If your objects are with full opacity, you don't get any different result in normal mode and just a color change effect in the other modes. You need transparency or the gradients for exact blending. And I think you need some gradient effects to blend this two objects. I suggest you to study and work with gradient tool and linear gradient mode to achieve this effect. You can define gradients for each objects and then you can use blend tool.

Gradients: http://tavmjong.free.fr/INKSCAPE/MANUAL ... -Gradients
Blend: http://tavmjong.free.fr/INKSCAPE/MANUAL ... ters-Blend

PS: You need to select two objects or an object with a background in order to use blend effect.

Ok, thank you!