The black rat snake is a highly variable snake, and this has alot to do with the locale of the snakes.
I grew up in MO and there the black rats are jet black as adults. Some still have a slight pattern with some red or white bleeding up between the side scales from the belly.
Now you get over here in KY were I currently reside, and the black rats maintain pattern and coloration througout their entire life. The snakes up in IL, IN, OH and down in TN have the same basic look, but each have their own locale specific look.
Many believe this is due to the range overlaps of the black rat and the gray rat. If you look at a range map for both snakes, you will see where the two ranges have a very broad overlap. And this happens right in the states I mentioned above.
What you are looking for are a pair of locale specific snakes that come from the range where there is no natural intergradation of the two snakes. I would suggest looking for a black rat whose lineage is from NE portion of the states or from the midwest, MO, NE OK, Iowa, and maybe NW IN.
Here are a few pics for you....
This one is a KY locale black rat, 4 years old and over 6'. He is not what comes to mind when most people think of black rat, however, this is pretty representative of what you will find here in KY!! Although he does have a higher amount of yellow than most.

Now here is a black rat snake from NW MO. This is the male, and probably more representative of what you consider to be a black rat....Jet black

And the female. They both came from the exact same locale, and she has more white bleeding up on her sides than the male does, however, she has gotten darker since this pic was taken. Notice the high amount of reds inbetween her side scales. This is a common occurance of most black rat snakes that are jet black. Most have lots of reds between the scales, and isnt really noticeable until after a feed or when they are coiled up. I have found specimens that have very high amounts of reds on the belly and bleeding up the sides. These, IMHO, are very attractive.

I will hopefully be breeding these two next year.
Hope this helps in clariying some misconceptions for you.
Brian
>>I grew up in Florida and have lived most of my life in the southeast. I have seen and caught a few snakes that were clearly "black rat snakes". They are solid black above with a white chin. As hatchlings they have patterns with saddles.
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>>Lately I have been thinking of getting several snakes with a "black" theme: black kings, black milks, black pines, indigos (don't quibble about whether those are actually black, it is my project).
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>>Anyway, I see "black rats" advertised in the classifieds. When I look them up or ask to see pictures of the parents, they look like gray rats to me. They still have saddles clearly visible.
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>>I am looking for a pair or 2.2 of solid black as adults black rat snakes.
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>>But also, I suppose I am asking the difference in black vs. gray rats which I thought I knew until now when this strange project came to me.
>>
>>Can anyone help educate me?
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RATS
2.0 Corn snakes "Warpath" & "Thunderbird" (KY locale)
1.0 Black rat snake "Havok" (KY locale)
1.1 Black rat snakes "Reaper and Mystique" (MO locale)
1.0 Albino Black rat snake "Malakai" (Dwight Good stock)
1.0 Everglades rat snake "Deadpool" (Dwight Good stock)
0.1 Greenish rat snake "Rogue" (Dwight Good stock)
1.0 Great plains rat snake "Reign Fire" (TX locale)
1.0 Grey rat snake "Punisher" (White oak phase)(Dwight Good stock)
RACERS
1.0 Eastern Yellow Belly racer "Nightcrawler" (MO locale)
KINGS
1.1 California king snake "Bandit" & "Moonstar" (Coastal phase)
1.1 Prairie king snakes "Bishop" & "Askani" (KY locale)
0.1 Black king snake "Domino" (KY locale)
1.1 Desert Kingsnakes "Gambit" & "Psylocke"
0.1 Florida Kingsnake "Shard"
MILKS
1.0 Eastern/red Milk intergrade "Cable" (KY locale)
1.0 Eastern/Red Milk intergrade "Omega Red" (KY locale)
BULLS/GOPHERS/PINES
0.1 Sonoran Gopher "Husk"
Good luck and Happy Herping
Brian