The bracha is that Ya'akov is speaking to Reuven and Shimon to state that in spite of what they did wrong, they are still his children and merit the general blessing that he had given to all 12 of them. Additionally, he pointed out (only now) what they had done wrong so that they could fix it and do better. He also pointed it out only now so that they could see that he was saying it from completely pure motives, now that he was about to die.
Note that Levi took this to heart and made his trait of zealousness a blessing to serve Hashem.
Reuven took it to heart and became the tribe that was known for Teshuvah.
Moshe Rabbeinu did not include Shimon because he still had the problem.
Vay'chi 49:28
כָּל אֵלֶּה שִׁבְטֵי יִשְׂרָאֵל שְׁנֵים עָשָׂר וְזֹאת אֲשֶׁר דִּבֶּר
לָהֶם אֲבִיהֶם וַיְבָרֶךְ אוֹתָם אִישׁ אֲשֶׁר כְּבִרְכָתוֹ בֵּרַךְ
אֹתָם:
All these are the twelve tribes of Israel, and this is what their
father spoke to them and blessed them; each man, according to his
blessing, he blessed them.
Rashi
וזאת אשר דבר להם אביהם ויברך אותם: והלא יש מהם שלא ברכם אלא קינטרן.
אלא כך פירושו וזאת אשר דבר להם אביהם מה שנאמר בענין. יכול שלא ברך
לראובן שמעון ולוי, תלמוד לומר ויברך אותם, כולם במשמע:
and this is what their father spoke to them and blessed them: Now is it not so that some of them he did not bless, but [in fact] chided?
Rather, this is what is intended: And this is what their father spoke
to them-what is related in this section. One might think that he did
not bless Reuben, Simeon, and Levi. Therefore, Scripture states: and
he blessed them, meaning all of them. — [From Pesikta Rabbathi 7]
איש אשר כברכתו: ברכה העתידה לבא על כל אחד ואחד:
according to his blessing: With the blessing destined to befall each of them.
ברך אותם: לא היה לו לומר אלא איש אשר כברכתו ברך אותו, מה תלמוד לומר
ברך אותם, לפי שנתן ליהודה גבורתו של אריה, ולבנימין חטיפתו של זאב,
ולנפתלי קלותו של איל, יכול שלא כללן בכל הברכות, תלמוד לומר ברך אותם:
he blessed them: Scripture should have said, “each man, according to his blessing, he blessed him.” Why does Scripture say, “he blessed
them” ? Since he (Jacob) bestowed upon Judah the might of a lion, and
upon Benjamin the power to seize like a wolf, and upon Naphtali the
fleetness of a gazelle, I might think that he did not include all of
them in all the blessings. Therefore, Scripture states:“he blessed
them.” - [From Tanchuam Vayechi 16]
The bracha of Yissaschar is indeed a blessing speaking of his dedication to the Torah and his accepting willingly the burden of study.
Vay'chi 49:14
יששכר חמור גרם: חמור בעל עצמות, סובל עול תורה כחמור חזק שמטעינין אותו
משאוי כבד:
Issachar is a bony donkey: Heb. חִמֹר גָרֶם, a bony donkey. He bears the yoke
of the Torah, like a strong donkey which is laden with
a heavy burden. — [From Gen. Rabbah 99: 9]
רבץ בין המשפתים: כחמור המהלך ביום ובלילה ואין לו לינה בבית וכשהוא רוצה
לנוח רובץ לו בין התחומין בתחומי העיירות שמוליך שם פרקמטיא:
lying between the boundaries: like a donkey, which travels day and
night and does not lodge in a house, but when it lies down to rest, it
lies between the boundaries, in the boundaries of the towns where it
transports merchandise. — [From Zohar vol. 1, 242a]