Only blend eyeshadows where two colors meet, or around the very edge of something you want to soften/blur. Make sure the pressure you are applying is very gentle, you need to hardly touch the surface of your skin and if the bristles of your brush are all splayed out you are more likely to get a very uneven blend or blend too much away. Sometimes all you need to do is pat the brush ontop of the area you want blended, like a stippling motion.

ABH's formula is notorious for being very powdery and prone to muddying up. Build the colors up very slowly, tap your brush off before applying, use small brushes and blend gently only where absolutely needed to avoid that.