To get this database setup, which is correct working the assocs would be:
- Donor belongsTo BloodType
- Donor belongsTo DonorType
- BloodType hasMany Donor
- DonorType hasMany Donor
But your foreign keys are wrong: Follow the CakePHP conventions!
Model fields are supposed to be lower cased and underscored, PKs are expected to be just id and FKs are the model name of the assoc, singular, underscored with suffix _id. The PKs in the donors table are right.
Why are you even changing your own convention? DonorId vs blood_group_id as PKs? However, if you want to cause a mess name them like you want but you'll have to declare them explicitly then everywhere. See linking models.
I recommend you to do the blog tutorial before messing with the framework to get a real project done.