The gilyon maharsha (both on the gemara and in y.d. 198:17) references the shevus ya'acov (69) who asks that a halacha in the rama seems to contradict our gemara. The gemara says that the kohanim would walk knee deep in blood because they would plug up the outlet of the courtyard to hold in all the blood of korbanos (according to R. Yehuda it was done so that they could do a zerika at the end of the day on all the korbanos slaughtered that day, but according to Rabbonon it was merely to show the "shevach" of the kohanim that they sacrificed so many korbanos on erev pesach). The gemara asks that the blood should be a chatziza between the feet of the kohein and the floor, and answers that since it is wet it is not a chatzitza. The implication of the gemara is that if the blood would dry on their skin causing a separation between their feet and the floor, it would constitute a chatzitzah. The problem is that the Rama paskens (y.d. 198:17) that if a woman's profession is to be a butcher or shochet the blood on her wouldn't be considered a chatzitza since people who have that profession aren't makpid about blood on themselves. Clearly, the Rama holds that even if the blood were to dry causing a separation between their skin and the mikva water, it still wouldn't be a chatzitzah. So, why by the kohanim does the gemara imply that it would be a chatzitza if it were dry since this is their profession?
The Shevus Ya'akov seems to conclude that paint on the skin isn't a chatzitza for a painter, but blood would be a chatzitza on the skin (just not on the clothing as the gemara and rashi imply later in zevachim 98b). However, this isn't sufficient to explain the Rama who says that blood isn't a chatzitza even on the skin for a shochet or butcher?
The sefer halachos ketanos paskens that a mohel who has blood on his hands can be tovel and it wouldn't be a chatzitzah citing our gemara that just as its a "shevach" for b'nei aharon to have blood on their feet, it is a shevach for a mohel to have the blood of the mitzvah on his hands. The Shevus Ya'akov asks that the gemara implies exactly the opposite that the blood would be a chatzitza after it dries?
I would like to suggest an approach to answer the question of the shevus yakov on the rama and on the halachos ketanos. There seems to be an inherent question on the gemara itself. The rule of chatzitza is that it is only a problem if the person is makpid about it. Since the gemara says that during the avoda it is a shevach to be knee deep in blood and they would set this up intentionally, there is certainly no hakpada against having the blood so why should it be considered a chatzitza (even if we don't accept the rama's chiddush that blood is never a chatzitza for a shochet, it certainly wouldn't be a chatzitza at a time where it is a shevach to be there)? The reason that something a person isn't makpid on isn't a chatzitza is because it becomes tafel to his body. We don't say that not being makpid is itself a rationale to not be regarded as a chatzitzah, it is merely a rationale to consider the item tafel to the body. This logic would apply to any type of substance that is attached to his body and he isn't makpid about. However, when the courtyard of the beis hamikdash is filled with blood, it is impossible that all the blood would be ta'fel to his body. Therefore, the only way that the enormous amount of blood wouldn't be considered a chatzitza is if it did not cause a blockage between the kohein and the floor, such as when it is moist. But if it were dry since there is so much blood it would be a chatzitza even if he wasn't makpid about it. However, for tevila any blood that one isn't makpid about on their skin such as a shochet or mohel, wouldn't be a chatzitza even when it's dry because it would be ta'fel to their body.
In my sefer Mayim Rabim (pg. 274) I quoted the sidrei tahara who answers the question differently. He says that the din that when the majority is covered it is a chatzitza even when one isn't makpid is judged by whatever are we require contact. Therefore, for tevila where the entire body needs contact, it is only considered a chatzitza when majority of the body has a chatzitza (or if he is makpid), but when all we require to touch the floor of the courtyard is the bottom of the foot, we judge majority by the foot. Even if we say that he isn't makpid it would at least be a chatzitza d'rabonon because of rubo she'eino makpid, and perhaps even m'doraysa according to those who hold that the entire body being covered would be a chatzitza even when she isn't makpid (since the entire bottom of the foot would be covered in blood).