These charmingly named undead have 10 RHD, medium size, and a less-than-stellar speed. The ability scores are downright insulting: its the average ooze's abysmal stats, now made worse by the lack of a constitution score.
Bloodrots have one natural slam attack that deals an unimpressive 1d6 damage and infects the target with Blood Fever: a curse that slowly damages constitution and charisma. There's some side effects that include liquefaction upon death and resistance to healing spells, but overall it's not that much better than any other disease ability.
The slam also come with improved grab and constrict, which are a lot less impressive once you realize this thing has +8 to grapple at level 10. For your information, a random sample of CR 8 monsters revealed grapple modifiers of +8 (on the rakshasa, an under-HDed caster) to +37 (monstrous scorpions), with most being somewhere in the 15-20 range.
Bloodrots also get two abilities related to their awful disease: Track Infected, which gives information about the position of creatures they've infected as long as they stay within a few miles, and Sanguineous Mount.
Now, I'll be fair, the latter is actually pretty cool. It's unique, flavorful, and surprisingly useful. Basically, the ability allows the bloodrot to enter the body of anything it's infected, which then nauseates (!) them for up to 24 hours, while also guarding the bloodrot from further harm.
The issue with what would otherwise be an awesome save-or-lose is that it requires at least one turn to set up, gives the target two saving throws to resist, and prevents the bloodrot from taking any more actions after it's nauseated a target. Also, you have to be a bloodrot to use it.
Finally, there's the Split ability, which I admit does a lot for making the monster version more threatening (I dare all DMs to try and infect the whole party with bloodrots). However, as something that's really not balanced for PCs, all it does here is add an asterisk to the LA of -0.
Tier, if I had to give one, would probably be around 5, and that's being generous.