"Most whitesided black ratsnakes I see are hybrids/intergrades of various species. No telling what might present itself if you have one of these"
Yeah, and for that matter, the same goes for the leucistic "Texas" Rat too. No telling if there isn't already leucistic Black Rat geneflow in there either, as there is really no meristical scalation difference between the two subspecies anyway. The only way to tell them apart is by their normal color/pattern phenotype anyway. And when that is not being displayed due to the leucistic gene being involved, who really knows what they really are over the years they have both been produced and crossed to whatever extent. Because I guarantee that many have over the years.
But in any case, crossing these two on PURPOSE would not be a good idea at all in my opinion. There are already more muddied-up types of snakes in the hobby than you could shake a stick at in 5 lifetimes, so no good reason to contribute more of these.. 
Also to the OP, the genetic combination you are contemplating wouldn't be seen at all, but the double het offspring when paired would create both types in the clutch. But these would be DEFINITE man-made crosses then as other's have said, and moving them would not be very appealing to most. And the other's that wouldn't mind don't care or know any better anyway to be quite honest. I would do as Allen suggested and do your very best to keep them as genuinely pure as possible, and not go crossing stuff on purpose to further muddy-up genepools more than they already are.........lord knows it is quite extensive already. 
~Doug
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"a snake in the grass is a GOOD thing" 
my website -Serpentine Specialties