Q:

Why does Ramanuja refer to the PAshupata sect as the sect of "Black faces"?

Ramanuja attributed this philosophy to the tradition of the Kalamukha(s), the sect of "Black Faces" to which Lakulisha belonged.
What was the reason for referring to it as KAlamukha? 
What were RAmanuja's views on the KApAlika and Pasupata sects and their doctrines? 

A:

Ramanujacharya does not call Pashupatas as Kalamukhas. In fact, he explicitly states that they are different Shaivite sects, in this section of his Sri Bhashya:

So far it has been shown that the doctrines of Kapila, Kanâda, Sugata, and the Arhat must be disregarded by men desirous of final beatitude; for those doctrines are all alike untenable and foreign to the Veda. The Sûtras now declare that, for the same reasons, the doctrine of Pasupati also has to be disregarded. The adherents of this view belong to four different classes--Kâpâlas, Kâlâmukhas, Pâsupatas, and Saivas. All of them hold fanciful theories of Reality which are in conflict with the Veda, and invent various means for attaining happiness in this life and the next. They maintain the general material cause and the operative cause to be distinct, and the latter cause to be constituted by Pasupati. They further hold the wearing of the six so-called 'mudrâ' badges and the like to be means to accomplish the highest end of man. Thus the Kâpâlas say, 'He who knows the true nature of the six mudrâs, who understands the highest mudrâ, meditating on himself as in the position called bhagâsana, reaches Nirvâna. The necklace, the golden ornament, the earring, the head-jewel, ashes, and the sacred thread are called the six mudrâs. He whose body is marked with these is not born here again.'--Similarly the Kâlâmukhas teach that the means for obtaining all desired results in this world as well as the next are constituted by certain practices--such as using a skull as a drinking vessel, smearing oneself with the ashes of a dead body, eating the flesh of such a body, carrying a heavy stick, setting up a liquor-jar and using it as a platform for making offerings to the gods, and the like. 'A bracelet made of Rudrâksha-seeds on the arm, matted hair on the head, a skull, smearing oneself with ashes, &c.'--all this is well known from the sacred writings of the Saivas. They also hold that by some special ceremonial performance men of different castes may become Brâhmanas and reach the highest âsrama: 'by merely entering on the initiatory ceremony (dîkshâ) a man becomes a Brâhmana at once; by undertaking the kâpâla rite a man becomes at once an ascetic.'

As far as how Ramanujacharya feels about Pashupatas, Kapalikas, and others, in this section of the Sri Bhashya he discusses how their beliefs and practices are criticized in the Brahma Sutras:

With regard to these views the Sûtra says 'of pati, on account of inappropriateness.' A 'not' has here to be supplied from Sûtra 32. The system of Pasupati has to be disregarded because it is inappropriate, i.e. because the different views and practices referred to are opposed to one another and in conflict with the Veda. The different practices enumerated above, the wearing of the six mudrâs and so on, are opposed to each other; and moreover the theoretical assumptions of those people, their forms of devotion and their practices, are in conflict with the Veda. For the Veda declares that Nârâyana who is the highest Brahman is alone the operative and the substantial cause of the world, 'Nârâyana is the highest Brahman, Nârâyana is the highest Reality, Nârâyana is the highest light, Nârâyana is the highest Self'; 'That thought, may I be many, may I grow forth' (Kh. Up. VI, 2, 3); 'He desired, may I be many, may I grow forth' (Taitt. Up. II, 6, 1), and so on. In the same way the texts declare meditation on the Supreme Person, who is the highest Brahman, to be the only meditation which effects final release; cp. 'I know that great Person of sunlike lustre beyond the darkness. A man who knows him passes over death; there is no other path to go' (Svet. Up. III, 8). And in the same way all texts agree in declaring that the works subserving the knowledge of Brahman are only those sacrificial and other works which the Veda enjoins on men in the different castes and stages of life: 'Him Brâhmanas seek to know by the study of the Veda, by sacrifice, by gifts, by penance, by fasting. Wishing for that world only, mendicants wander forth from their homes' (Bri. Up. XI, 4, 22). In some texts enjoining devout meditation, and so on, we indeed meet with terms such as Pragâpati, Siva, Indra, Âkâsa, Prâna, &c., but that these all refer to the supreme Reality established by the texts concerning Nârâyana--the aim of which texts it is to set forth the highest Reality in its purity--, we have already proved under I, 1, 30. In the same way we have proved under Sû. I, 1, 2 that in texts treating of the creation of the world, such as 'Being only this was in the beginning,' and the like, the words Being, Brahman, and so on, denote nobody else but Nârâyana, who is set forth as the universal creator in the account of creation given in the text, 'Alone indeed there was Nârâyana, not Brahmâ, not Isâna--he being alone did not rejoice' (Mahopanishad I).--As the Pasupati theory thus teaches principles, meditations and acts conflicting with the Veda, it must be disregarded.

In subsequent sections of the Sri Bhashya he discusses how the Brahma Sutras refute the belief of these Shaivite sects that Ishwara is the efficient cause but not the material cause of the Universe.

