When I've usually seen roasted barley and black barley from the same malter, the roasted barley is usually rated between 300l and 350l while the black barley is 500+l. This usually means the black barley will have a drier more charcoal flavor while the roasted barley is more coffee, and roasty. I do not believe either is malted. I see very dark malts listed as Black Patent or Black Malt, which is supposed to have a different flavor from the unmalted barleys despite being of similar color.
I'm assuming the OP is referring to Black Patent when he says Black Barley and based on that yes, there's a HUGE difference. Black Patent is very harsh, very bitter and in my opinion should be used very sparingly. Roasted Barley on the other hand has a yummy roasted grain flavor.
I just made a bulk purchase of several grains these two included and as I was moving them to their respective storage containers I munched on a few grains of each one. Next time you’re at your LHBS try a couple grains of each and believe me, you’ll see the difference. Roasted Barley tastes good and Black Patent will send you running for something to wash it down. I’m not saying you shouldn’t use Black Patent, it goes in all my stouts, it’s just extremely harsh.