for has_many we will use plural definitely
for belongs_to we will use singular
for ex:
has_many :users
belongs_to: user
But for has_and_belongs_to_many
has_and_belongs_to_many :user, join_table: :blocked_contacts
here it should be user or users
It should be
has_and_belongs_to_many :users, because it is a many-to-many relationship. You can find more info here.