Is it okay for Christians to deliberately blaspheme the Holy Spirit because the unpardonable sin was only for Israel?
No, Christians must never deliberately blaspheme the Holy Spirit. Although the unpardonable sin was spoken to Israel and does not fit church doctrine, the warning still stands: no one should presume on God's forgiveness. The proper response is to heed the warning, not exploit that distinction.
Les explains that although the unpardonable sin was addressed to the nation of Israel and is something that does not fit church doctrine, believers can still take a warning from it. He says he certainly does not tell people, "go out and blaspheme the Holy Spirit, because, after all, God will forgive you"—"No way." The passage states that all manner of sin and blasphemy shall be forgiven unto men, but blasphemy against the Holy Spirit shall not be forgiven, neither in this age (aion) nor in the age to come. Rather than pigeonholing the issue, he says to leave it where it belongs. He then points to a parable in Matthew 21 that explains it beautifully.