The Mechaber in Hilchos Tisha BeAv in 554 Seif 24 brings down, that anyone who does Melacha on Tisha BeAv doesnt see blessing from that work. I tutor a boy in Gemara, if I tutor him on Tisha BeAv, and we will learn stuff about the Aveilus, is that included in this " that anyone who does Melacha on Tisha BeAv doesnt see blessing from that work

ב"ה Shalom, The Mishna Brura 554:49 says, that the lack of Bracha according to the Gemara , is when one intentionally makes up to do work to distract himself from the Aveilut. Therefore, when you are tutoring, although it is work, you are teaching him about the Chorban and therefore not distracting your mind from the Aveilut. Therefore, it seems that tutoring about the Chorban does not come under the category of not seeing Bracha. I was instructed so by a prominent Rabbi. Even so, it is better to do so after Chatzot, after the Kinot have been said. On the other hand, there are those who are more stringent and rule not to receive such a salary on Tishah B'av. All the best