Why has Israel been blinded for the last 1900 years?
The context does not explain why Israel has been blinded for about 1900 years. It affirms the national spiritual blindness and that God is not through with His covenant people. It also traces a transition: God dealt predominantly with Jews until Acts, then Paul was sent to Gentiles after Jewish rejection.
Les explains that Romans 11 makes clear God is not finished with His covenant people Israel, even though they have spent roughly 1900 years in spiritual blindness and the nation as such is blinded. He notes that from Genesis 12, the call of Abram, all the way into Acts, God's dealings were predominantly Jew only, with exceptions like Jonah, Naaman, Rahab, and Ruth. After getting well into Acts, Paul is called aside and sent to the Gentiles. By the end of Acts, Paul is imprisoned in Rome, near martyrdom, yet writing prison epistles. He had already been among Gentiles for many years, but he always went first to the Jew in the synagogue, and when they rejected him, he turned to the Gentiles. This shows the transition while God remains faithful to Israel.