Blood hunting is an action that a commander can do in an owned province where they attempt to collect blood slaves as the cost of population and unrest.
In general you should aim to blood hunt provinces with around 5k population (less gives fewer slaves, more causes larger losses in taxes) and keep the unrest in those provinces low. Give all blood hunters sanguine dousing rods if available, an effective blood hunting level is 2 is significantly better than 1 and 3 is noticeably better than 2 but above 3 the benefit from additional levels of blood magic and dousing fall off.
There are three factors which influence the chance of success when blood hunting, Blood hunter's skill, province population, and unrest. These three factors are all treated as individual percent chances of success and can not go above 100%.
Firstly the hunter's blood magic and blood searcher bonus are added together to find their blood hunting level. They then have a 10+(40*lvl)% chance of success in this phase. For example any regular commander with no magic has a 10% chance of not failing to find slaves while an with 1 and a blood searcher of 1 has a 90% of success in this step of the process. If you gave the Ah Nakom a for an additional dowsing bonus they would have a 100% chance of success.
Secondly if the province's population is too small then the blood hunter might fail to find any slaves in the month. The percentage chance of success = province’s population / 50. With 0 population it is intuitive that there are no blood slaves to be found. The chance of success rises linearly up to 5,000 population. Beyond that point a higher population does not improve the chances of success. For example a small 2,500 population provinces would only give a 50% chance of passing this phase while a massive 35,000 population capital province would have the same 100% chance of success as a 5,000 population province.
Thirdly, unrest in a province can prevent a blood hunter from finding slaves. Percentage chance of failure = province unrest / 4. Thus the chance of failure rises linearly from 0% to 100% from 0 unrest to 400 unrest. Note that multiple blood hunters in a province create unrest so the first to hunt will have an better chance of success, cause unrest, and make a worse chance of success for those who follow even on the same turn. For more information see the unrest section.
To find your final chance of successful blood hunting simply multiply the three factors together. For example a lvl 2 blood mage with no dowsing bonus in a 4,000 population province with 40 unrest would have a 70%*80%*90%=50.4% chance of success.
Following a successful blood hunt you will get d6+blood hunting level+magic site frequency modifier slaves. As before, blood hunting level is blood magic level plus dowsing bonus. The magic site frequency modifier is 0.5 slaves per 5% the magic site frequency of the game deviates from 50. For example a 2) with a sanguine dowsing rod (+1 blood searcher bonus) in a game with a magic site frequency of 40 (standard for the late age) would find d6+2 (d6+3-1) blood slaves if successful.(
The population loss from blood hunting is 10 * ((slaves + 4)/5, rounded down).
Succeed or fail, blood hunting produces unrest. If the blood hunt succeeds then unrest in the province will increase by d(slaves x 3 +4). That’s a random number between 1 and three times the number of blood slaves found, plus 4. If the blood hunter did not find and slaves then there is a d6-1 increase in unrest.
The reduction in unrest caused by units with reduces unrest tag can remove unrest before it impacts income unlike all other ways to reduce unrest. There are a limited number of units with this trait, notably Ind's s and the s of LA Phlegra. How much unrest reduction you need to sustainably hunt a province without hurting its income will depend on the number and quality of the blood hunters, but with four (1) hunters with dowsing rods roughly 40 reduction is sufficient. With Phlegran priests that will require 20 com points, 1300 gold and 20 gold per turn in upkeep. With the bishop vicomte it takes 80 com points, 4600 gold and 153 gold per turn in upkeep for the bishop and 56 com points, 2240 gold and 75 gold a turn in upkeep for the viceroy primate.
All magic paths can be "Bootstrapped" into, starting off without access to the path and empowering, boosting, and summoning your way up to reasonable access to the path. Blood is a particularly easy path to bootstrap into because anyone can hunt for blood slaves, even if poorly.
Initial bootstrapping into blood can be done through a myriad of ways:
Regardless of the initial method, the goal is to produce at least one level 1 blood mage by which to gather more slaves and empower yet more blood mages with. Ideally this will also involve the use of a sanguine dowsing rod. According to blood hunting calculations in this spreadsheet by Mandarbmax, it is better to empower scouts up to blood 1 and then give them blood thorns if avaliable rather than just empowering them as this results in more slaves per turn per initial cost and more slaves per unrest generated. This conclusion is valid both with and without sanguine dowsing rods. Summoning vampires using is significantly more efficient than empowering scouts.
An analysis of bootstrapping options for getting into blood as well as the results of using multiple blood hunters in a single province. If copied or downloaded the spreadsheet can be modified with different input variables.
Dominions 5 game manual. Blood hunting is addressed on page 64.