whilst that is true - they set out and intend to offer a uniform product across all their stores. Therefore by necessity bulk buying and blending from a range of inexpensive beans generally means a more boring cup of coffee. As for the dark roast that stems more from the tradition of the Itialian style roast for the production of Espresso which given the machines most outlets use - is the base for all coffee drinks. In reality all commercial roasters are the same or similar - but I am well informed that Costa takes more care than others, and sets higher standards than others.
Different beans (not necessarily better) roasted lighter can yield a more interesting drink, but this is highly dependent on the bean, and where it is grown. I regularly roast well into second crack very dark (but not City or Itialian) to produced a well flavoured unburnt coffee. Under roasted good beans can taste far worse than over roasted.
The extraction method makes a huge amount of difference to the flavour profile.