The gemara assumes that when a person is accustomed to doing something that causes stains, they aren't makpid about those stains and therefore they aren't considered a chatzitza. Therefore, blood isn't considered a chatzitza on the clothes of a butcher and wax isn't considered a chatzitza on the clothing of a wax merchant. But, the gemara is clear that wax would be a chatzitza on butcher and blood would be chatzitza on the clothes of a wax merchant, because only the exact item that they are involved in wouldn't be a chatzitza. The gemara then questions when one is a butcher and a wax merchant and has both blood and wax on his clothes, is it considered a chatzitzah. Perhaps each one independently isn't a chatzitza but the combination of the two at the same time is significantly degrading and qualifies as a chatzitza, or not? The gemara leaves with a teiku.
The Rama (Y.D. 198:17) writes that one who is a shochet or butcher who's hands are always bloody, blood would not be a chatzitza for them since most people of that type aren't makpid about blood. The Beis Hillel (gilyon of shulchan aruch) says that if a woman was a writer and dyer, and she has both ink and dye on her when she goes to the mikva, she must go again. The rationale is that the gemara remains with a teiku about a combination of two substances even when the person is involved in both occupations, and therefore it is necessary to be machmir. The Sidrei Tahara (Shiyurei Tahara 34) quotes this agrees with the Beis Hillel because the Rambam (Mikvaos perek 3) concludes that it is a safeik, and so does the magen avrohom (161:7). It seems that the poskim are machmir even though the chatzitza is only on part of her body or even a small amount on part of her hand, since we have a safeik whether she is makpid. Why don't we say that since it is only a mi'ut which is d'rabonon, and we have a safeik about whether she is makpid, it would be a safeik d'rabonon and we can be meikel to say that it doesn't qualify as a chatzitza?