For reference, it takes a LOT of O'ghomoro berries to reach Royal Blue in the manner Sayori described. What is useful to know is that Midnight Blue is a color that is very close to black. When thinking in terms or RGB values, Midnight Blue is reachable from Soot Black by feeding it 3-5 berries.

So, Sayori's method seems perfectly viable if you have 80 or so berries to spare. These would all be fed in succession, and then the two apples and one berry.

An alternate method involves all three darkening fruits, fed in this relatively precise order. This is starting from Desert Yellow!

20 O'ghomoro Berries(Deepen Blue)
20 Mamook Pears(Deepen Green) *You can also do this by alternating between green and blue 20 times. If you don't alternate, you must do Blue first.
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15-17 times, Alternate between Xelphotal Apples(Deepen Red) and O'ghomoro Berries(Deepen Blue). These really should be alternated, but you can feed Red first and Blue second if you don't want to. It might screw things up though.
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4 times, Alternate between Deepen Red and Deepen Green. As with the first set, you can feed 4 Red and then 4 Green without mucking it up.

If RNG smiles upon you(as the strength of the fruit's effects are slightly random), you will have a dark blue color when you're done with this. If it isn't Midnight Blue, you'll be close.