Why dont we make a special blessing on eating the afikomen as we do for example on eating the bitter herbs?

Shalom, Thank you for your question. There is actually quite an argument amongst early Rabbis as to what exactly the mitzvah of eating the Afikoman is. Rashi, and the Rashbam, believe that the mitzvah of eating Matzah in Seder night, which is a Torah level mitzah even today when we do not have the korban Pessach, is fulfilled at the end of the meal, when eating the Afikoman. This sounds strange, as we say the blessing of eating the Matzah (al acilat matzah) at the beginning of the meal, together with HaMotzi. None the less, they believe, that just as when we ate the Korban Pesach, together with the Matzah and Maror (bitter herbs) that was eaten at the end of the meal (after the korban Chagigah), so too, today, the mitzah of matzah is fulfilled at the end of the meal – despite the fact that the bracha was said at the beginning of the meal. (It’s beyond the scope of my answer here to get into all the details of how they believe a bracha can be separated from the act of the mitzvah, suffice to say that they have an answer). Most other Rabbis believe that the Mitzah Matza is the one we eat at the beginning of the meal. The Afikoman is “only” a Rabbinic practice in order that the last taste left in our mouth be matzah – just as in Temple times the last taste had to be the Korban Pessach. Because this is not a separate mitzah in and of itself, but only to guarantee that the taste on matzah stays in our mouths, it did not merit it’s own bracha. The first bracha we said on eating Matzah will “cover” this eating also. I hope this is of some help, Blessings.