Multiple election officials confirmed that filling out ballots with blue or black Sharpie is acceptable, and is actually preferred.
Some in-person voters complained that election officials forced them to use Sharpie pens at their polling place and that the ink spoiled and invalidated their ballots. However, that is not true. In some cases, Sharpies are actually recommended as the best type of pen to use on ballots, and election officials have taken steps to guard against ink “bleed-through” on ballots. There is no evidence that elections officials used Sharpies to invalidate ballots.
Sharpies are actually the preferred in some states, and they are valid in states like Michigan, Pennsylvania, and Arizona.