Please blot me out of your book that you have written. Did Moses really mean it?

The Israelites made a golden calf to replace the LORD in Ex 32:

30 The next day Moses said to the people, “You have sinned a great sin. And now I will go up to the LORD; perhaps I can make atonement for your sin.” 31 So Moses returned to the LORD and said, “Alas, this people has sinned a great sin. They have made for themselves gods of gold. 32 But now, if you will forgive their sin—but if not, please blot me out of your book that you have written.”

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Moses interceded for the Israelites. Did he mean that literally?

Perhaps not, I think it was a figure of speech. It demonstrated his compassion for the people before God in a manner of pleading. He knew the LORD would not accept that:

33 But the LORD said to Moses, “Whoever has sinned against me, I will blot out of my book. 34 But now go, lead the people to the place about which I have spoken to you; behold, my angel shall go before you. Nevertheless, in the day when I visit, I will visit their sin upon them.”

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No deal.

35 Then the LORD sent a plague on the people, because they made the calf, the one that Aaron made.

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This kind of pleading was not uncommon in ancient Chinese culture, when an innocent person pleaded for his guilty relative before an authority, hoping for mercy. It was a Confucian honorable act.