We Pasken that a Kesem (blood stain that is presumed to be uterine blood) that is found on a colored garment is not Tamei.
Rashi on Niddah 61b says that Chazal never forbade Kesamim on colored garments because it is hard to recognize the color and tell whether it is a Tamei shade if the garment is colored.
להקל על כתמיהן - שאין הכתם ניכר בו כמראה דם גמור:
Based on this would a Kesem found on a tattoo be Tahor as well?