Damage is a characteristic, thus you would use the modify characteristics rule.
7. WEAPONS
In the condensed datasheets, weapons are given a number that corresponds to annotated pictures of the miniature, rather than being named. Weapons are described with the following characteristics:
Range: How far the weapon can shoot. Weapons with a range of ‘Melee’ are melee weapons and can only be used in hand-to-hand combat. All other weapons are ranged weapons. Some weapons have a minimum and maximum range, for example 6"-48"; such weapons cannot target units that are wholly within the shorter range.
Type: These are all explained under the Shooting and Fight phases of the basic rules. These types are denoted by symbols on the condensed datasheet.
Strength (S): How likely the weapon is to wound a foe. If a weapon’s Strength lists ‘User’, it is equal to the bearer’s Strength characteristic. If a weapon lists a modifier (e.g. ‘+1’ or ‘x2’), modify the bearer’s Strength characteristic as shown (e.g. if a weapon’s Strength was ‘x2’, and the bearer had a Strength of 6, that weapon has a Strength of 12).
Armour Penetration (AP): How good the weapon’s attacks are at getting through armour.
Damage (D): The amount of damage inflicted by a successful wound.
If the damage is 2, and virulent blessing is active, and you roll a 7+, the damage would be 5.
MODIFYING CHARACTERISTICS
Many rules modify the characteristics of models and weapons. All modifiers to a characteristic are cumulative; you must apply division modifiers before applying multiplication modifiers, and before applying addition and then subtraction modifiers.
LOCUS OF VIRULENCE
Each time you make a wound roll of 6+ for a NURGLE DAEMON unit within 6" of any friendly model with the Locus of Virulence, that attack inflicts 1 additional damage.
VIRULENT BLESSING
Virulent Blessing has a warp charge value of 7. If manifested, select a friendly NURGLE DAEMON unit within 18" of the psyker. Add 1 to wound rolls made for that unit in the Fight phase. Furthermore, wound rolls of 7+ made for that unit in the Fight phase inflict double damage.