Blends of coffee are bad because they incorporate beans from different areas in the blend which increases the chances that there will be moldy beans. I want to make clear that, plain and simple, blending high quality beans with low quality beans is done to make a bag of coffee more profitable. It’s an easy way to make a cheap bag of coffee into a much more expensive bag simply by adding 10-20% of high quality beans.
If the fillers are very low quality beans, the fine aroma of the original beans is overtaken by the imperfections of the low end fillers. That is when your brand name coffee needs to be sprayed with chemically produced coffee aroma in order to be appealing.
Also, coffee this bad will generate a harsh feeling in your throat and will leave a lingering bad aftertaste that ranges between bitter and metallic. Generally, makers of this coffee will resort to a very dark roast in order to create the perception of "richness" in the cup while narrowing the range of aroma & flavor and hiding the low quality.