This is not a permanent problem after neutering.In large breeds and those who have surgery after puberty possibility of bleeding with urination ( hematuria) specially at the end of urination as blood spots is more.Also bruising, skin color changing ( redness) around the skin incision and superficial skin inflammation of the scrotum can be seen.
Reason : in large breeds like Labrador dogs and adult dogs vascular tissues of testicles are very congested.In this surgery these vascular tissues are blocked by surgeon.Therefore blood pressure in the blocked arteries and ducts are increased which lead to edema of the skin of the scrotum and sometimes a little bit bleeding in ductus deferens .
If your dog  shows these signs after castration, refer him to your vet to manage it.