There are four coat colors in the Neapolitan Mastiff,and each of the four colors can have brindling. There is Black, Blue, Mahogany, & Tawny.
The genotype of a black Neo must be by definition BBDd or BBDD or BbDD or BbDd.
The genotype of a blue Neo must be by definition BBdd or Bbdd.
The genotype of a mahogany Neo must be bbDd or bbDD.
The genotype of a tawny Neo is only bbdd.
Breeding blue to blue will never give black, but it may give tawny,but will never make black or mahogany.
Breeding tawny to tawny will always produce tawny. Breeding black to black or to any of the other colors can produce all four colors, depending on genotypes involved.
Mahogany bred to blue can give all four colors, depending on the genotype of the two individuals.
Black bred to black,mahogany,blue,or tawny can produce all four colors, depending on the genotype of the blacks.

None of these four colors should be considered rare. You can get each of the colors easily if you breed for them.

Here are some great articles on color from some of the message boards, these are all past discussions.
http://www.neapolitan.org/phpBB/viewtopic.php?t=3689&highlight=bbdd

http://www.neapolitan.org/phpBB/viewtopic.php?t=6926&highlight=bbdd

http://www.neapolitan.org/phpBB/viewtopic.php?t=6842&highlight=bbdd

http://www.neapolitan.org/phpBB/viewtopic.php?t=5045&highlight=bbdd

http://neapolitanworld.com/community/modules.php?name=Forums&file=viewtopic&t=419&postdays=0&postorder=asc&start=30