Indeed.
Short of any scientific papers on the matter (which may exist) evidence suggests that 'black voices' sometimes sound deeper is simply down to body size,
hence a large black man and a large white man are indistinguishable other than any social/cultural accent.
As quoted before, this article discusses voice physiology:
http://io9.com/5928125/do-people-of-different-races-have-different-voices
This post is from a professional singer:
http://www.quora.com/Why-does-an-African-American-persons-voice-have-a-different-timbre-from-a-Caucasian-voice
An article by a 'white sounding' black woman
http://www.nerve.com/features/being-black-and-sounding-white-and-why-no-one-speaks-correctly-anyway
Variations in the amount of melanin are not contributing to vocal tone compared to accent and social speech conventions.
So it makes as much sense to say - 'I want a voice actor with a "black voice"'
as it does to say - 'I want a voice actor with a "gay voice"'
In other words it doesn't make sense for sound.
So long as the voice actors are of medium build, their voices are capable of generic speech.
However...
What may be causing the issue is that Brian T Delaney and Courtenay Taylor are known individuals of known race,
and an initial reaction may be 'I want a black actor voicing my black guy'.
Frankly, after a couple of hours running around the wilderness, let alone 100,
I think the voices will fit your character perfectly well.