You should read the whole chapter 11 of Romans to understand that Gentiles were not Jews or descendants of Jacob.
Jews blindness was prophesied in the Book of Acts chapter 13.
Ac 13:6 And when they had gone through the whole island unto Paphos, they found a certain sorcerer, a false prophet, a Jew/Elymas, whose name was Bar-jesus;
Sergius Paulus a Gentile wanted to hear the word of God [Acts 13:7] from Paul and Barnabas but Elymas, a Jew, “withstood them, seeking to turn away the deputy from the faith. –Acts 13:8”
Ac 13:9 Then Saul, (who also is called Paul,) filled with the Holy Ghost, set his eyes on him,
Ac 13:10 And said, O full of all subtlety and all mischief, thou child of the devil, thou enemy of all righteousness, wilt thou not cease to pervert the right ways of the Lord?
Ac 13:11 And now, behold, the hand of the Lord is upon thee, and thou shalt be blind, not seeing the sun for a season. And immediately there fell on him a mist and a darkness; and he went about seeking some to lead him by the hand.
This is a "type" of the "Jews" rejecting Christ again for the second time:
1st was during Christ earthly ministry,
2nd was after the resurrection of the Lord Jesus Christ.
So, what happened here was, the Jews/TYPE/ELYMAS, because of UNBELIEF, was, “and thou shalt be blind, not seeing the sun for a season.” Compare this to Romans 11:7
Now, see what happened to Sergius Paulus/TYPE/Gentiles.
Ac 13:12 Then the deputy, when he saw what was done, believed, being astonished at the doctrine of the Lord.
This was the beginning of the salvation of the Gentiles by Paul and we can read this in details in Romans 11.
IN THE BOOK OF ROMANS CHAPTER 11
Ro 11:7 What then? Israel hath not obtained that which he seeketh for; but the election hath obtained it, and the rest were blinded Jews/TYPE/ELYMAS –Acts 13.
Ro 11:8 (According as it is written, God hath given them the spirit of slumber, eyes that they should not see, and ears that they should not hear unto this day.
Ro 11:9 And David saith, Let their table be made a snare, and a trap, and a stumblingblock, and a recompence unto them:
Ro 11:10 Let their eyes be darkened, that they may not see, and bow down their back always.
Ro 11:11 I say then, Have they stumbled that they should fall? God forbid: but rather through their fall salvation is come unto the Gentiles/TYPE/SERGIUS PAULUS –Acts 13, for to provoke them to jealousy.
Paul’s warning to the Gentiles about being arrogant:
Ro 11:19 Thou wilt say then, The branches were broken off, that I might be grafted in.
Ro 11:20 Well; because of unbelief they were broken off, and thou standest by faith. Be not highminded/ARROGANT, but fear:
NOW, we will see the word “MYSTERY/MUSTERION” IN THE FF VERSE, AND THIS IS VERY INTERESTING.
In the NT it denotes, not the mysterious as with the English word, but that which, being outside the range of unassisted natural apprehension, can be made known only by Divine revelation, and is made known in an manner and at a time/season [Acts 13] appointed by God, and to those only who are illumined by His Spirit. –W.E. Vine.
IOW, this mystery/musterion about the temporary blindness of Israel that was prophesied in Acts 13 was revealed to the Gentiles, but Paul warned the Gentiles not to be arrogant about this revelation or “mystery/musterion” about the blindness of Israel because it’s only “for a season –Acts 13:11” “until the fullness of the Gentiles be come in. –Rom. 11:25”
Ro 11:25 For I would not, brethren, that ye should be IGNORANT [or lack of knowledge] of this mystery/MUSTERION, lest ye should be wise in your own conceits [ARROGANT like Elymas]; that blindness/Jews/TYPE/ELYMAS in part is happened to Israel, until the fullness of the Gentiles be come in.
“Blindness in part is happened to Israel, until the fullness of the Gentiles be come in” Remember in Acts 13:11”not seeing the sun for a season.” Meaning Israel’s blindness is not permanent “until the fullness of the Gentiles be come in” then “all Israel shall be saved: Romans 11:26”
Like Elymas [Jews/TYPE/ELYMAS], you were blinded also. You were prophesied to believe whatever you want to believe. You can inject all your theology, all these would not matter because you were prophesied not to see and not to hear the truth, "until the fullness of the Gentiles/TYPE/SERGIUS PAULUS be come in", I.E., if you are a Jew of course. Paul was talking about the literal Gentiles and not the “[Israelite] gentiles” you are talking about. There is no such thing as “[Israelite] gentiles”
Now, the question is who are the true Semitic Jews today?
Rev 14:1 Then I looked, and there before me was the Lamb, standing on Mount Zion, and with him 144,000 who had his name and his Father’s name written on their foreheads.
Rev 7:4
Then I heard the number of those who were sealed: 144,000 from all the tribes of Israel.
You read this 144,000 came from the 12 tribes of Israel. These are the true Semitic Jews and the rest of mankind are Gentiles.
Rev 14:4 These, the 144,000 Semitic Jews, are those who did not defile themselves with women, for they kept themselves pure [Semitically pure]. They follow the Lamb wherever he goes. They were purchased from among men and offered as firstfruits to God and the Lamb.
Compare these verses from your statement here The Babylonian exile diluted the racial purity of the 12 tribes of Israel. IOW, during the Diaspora, the Jews were marrying or interbreeding with the Gentiles and therefore un-preserving the racial purity of the Semitic people or they are being hybridized.
The black, Chinese, and Spanish were proselytes to Judaism. You can tell the Black/Jews, Chinese/Jews, and Spanish/Jews have some physical similarities with their Gentiles host-countries.