Its actually better if Republicans in blue states refuse to be counted. Eventually, the states will have to redistrict based on the new numbers. They will balance their districts to garner maximum democrat seats based on the new numbers. But now, the unaccounted for Republicans WILL turn out to vote, and should be able to swing some districts, because they went stealth during the census. They just have to remain uncooperative in the follow up surveys.
Assuming the census bureau would not make up a number to estimate the number of people who were not counted.