Thanks for your quick and enlightening answer.
Meanwhile I tested a different printer (a 3880) and I obtained the very same results with that same paper as with the 4900. As you mention, the density reaches a more or less plateau at values 65-70-75 and then the values start to get lighter again from 80 and on to 100. I assume that means I can select a value of ink limit for the blacks of around 60, with a black boost of 65.
Now, based on this effect, I do not understand why on his (great otherwise) manual, Tom Moore mentions using value of 100 for the black boost, which for instance for this paper I was testing today would provide a way lighter value than 65. More sense has the recommendation of Diallo Photography, where black boost is given a value of black limit + 5 more or less, or a value well in the plateau as you mention?
??
Rafael