If box 9 is blank on my W-2, does that mean I used the standard deduction or did I itemize?

Your W2 does not show which deduction you used.  You mean  your 2019 1040 line 9?   There should be an amount on line 9.  Unless you filed Married Filing Separate and your spouse itemized.  

Your W-2 has absolutely no relevance on whether or not you had the Standard Deduction or the Itemized Deduction on your tax return.

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Box 9 of the W-2 is always blank.  Box 9 currently has no purpose.  It was used in the past for something unrelated to most people's tax returns. 

For 2020, the standard or itemized deduction is on line 12 of form 1040.