Only by tasting a variety of dark-roasted blends from a variety of coffee roasters can you determine whether you prefer your espresso coffee roasted in a style that leans toward the lighter end of the espresso spectrum, with some of the acidy notes still discernible, or toward the darker end, where the bittersweet tones completely dominate. You may even find that you prefer a distinctly acidy style or (particularly if you liked the coffee you tasted in northern France) a very dark, almost black, charred-tasting style.
Your choice will be complicated because different blends of raw beans respond differently to the impact of the roast. Espresso blends in the classic northern Italian style, like the famous Illy Caffe, combine coffees from Brazil that are naturally sweet, low-acid, and full-bodied. These Brazil beans need only a moderately dark roast to muffle their already mild acidity and develop their inherent sweetness. On the other hand, many West-Coast American espresso blends are heavy with powerful, high-grown, brightly acidy beans from Central America that literally need to be tamed by a roast dark sufficiently to mute their powerful acidity.
Furthermore, two dark-roasted coffees may look the same but taste radically different depending on how tactfully the roast was handled. The trick to roasting coffee dark without losing flavor is moderating the temperature inside the roasting chamber toward the climax of the roast so the sugars in the bean caramelize rather than burn. Coffees in which the sugars have been burned taste thin, bitter, and carbony, hardly positive characteristics for espresso. If you taste an espresso with those characteristics, try a coffee from a different roasting company.