If you're sticking to style...
The schwarzbiers I've liked in the past are the ones where you can taste the roast. I've always wondered what's the point of this style and the Black IPA one. Why make a black version if you can't taste the roast? But that's my biases showing.
From the Black IPA BJCP guidelines:
Dark malt flavorsare low to medium-low; restrained chocolate or coffee flavors may be present, but the roasted notes should not be intense, ashy, or burnt, and should not clash with the hops.
And the Schwarzbier:
Light to moderate malt flavor, which can have a clean, neutral character to a moderately rich, bread-malty quality. Light to moderate roasted malt flavors can give a bitter-chocolate palate that lasts into the finish, but which are never burnt.
I think the point is to avoid having much of the dark-burnt quality because it can interfere with or detract from the fundamental characteristics of the style - intense hop flavor in an IPA and smooth, full malt character in a Lager. Those hard-roasted notes may work in stouts and some porters perfectly because it's a main focus of the flavor.
OTOH, if there's not a little bitterness from the roast and gets all it's color from dark crystal malt, it can just get a cloying, raisin/molasses flavor that doesn't balance as well. To me, when the flavor comes off as complex and balanced, it really works. Like jmcnamara notes, it's hard to know exactly where it starts being a stout. Maybe you think you're drinking a stout, you've lost the identity of the underlying style and gone too far.
