Or did they just view Islam as a great big monolithic bloc?
While it's widely imagined to go back to the earliest days of Islam, the division between is much later. Shi'a means "faction, party, tendency" and of course is Shi'at 'Ali--the faction of Ali--after Ali ibn Abi Talib, Mohammed's cousin and son-in-law, who died in 661 CE. However, the real "schism" wasn't until the rise of the apocalyptically influenced (and ardently Shi'i) Safavid Dynasty in Persia in 1501. The first crusade was proclaimed in 1095, and the phenomenon petered out, to my understanding, in the 15th century, before the rise of the Safavids.
So, what do I mean about Shi'a Islam not being really divided before that? There are certainly Shi'as throughout Islamic history, and people who have Shi'a like factions, but it's important to recognize that this is a tendency rather than a sect. It's hard to trace out clear lines of specifically Shi'a mosques or even, in most cases, continuous Shi'a communities from the earliest days to the rise of the Safavids. The Fatimids, for example, we could call Isma'ili Shi'a and had a Caliphate from 909 CE to 1171, and in fact drew some of their political legitimacy from Shi'a logic (Shi'as believe that the Caliph and Imam should be a direct descendent of Muhammed through Ali's line). However, while the Fatimid ruling class agreed with this logic, it's really unclear how much this "Shi'a" vs. "non-Shi'a" logic at the elite level affected Fatimid society at large. In fact, Fatimids are famous for having an administration filled with Shi'as, Sunnis, Christians, and Jews. Even within Fatimid Caliphate, there were splits in ideas about who should rule.
So Sunni is Islam has four major schools of jurisprudence, right? Right up through the 16th century there were Ottoman debates (Ottomans being both the [Sunni] Caliphs and the greatest Muslim power at the time) about whether Shi'a (particularly the type of Twelver Shi'a popular in Lebanon and later in Safavid Persia) was part of the Shafi'i school, or whether Shi'a was its own, totally legitimate fifth school. There were a variety of Shi'a sects and religious orders, and a variety of peasants and herdsmen (particularly in Lebanon and the area where Turkey, Iran, and Iraq come together) who gave particularly importance to Ali and his family, but Shi'a weren't really treated as a class apart until the rise of the Safavids created the Shi'a Sunni divide as a clear political issue (the Safavids vs. the Ottomans) rather than merely an academic one or a matter emphasis by the common people. If you look at some of the people called Shi'a today, the Alevis of Turkey for instance, you can see that they place particularly importance on 'Ali and exist outside the normative framework of Sunni Islam (many, for instance, don't see it as incumbent to keep the fast of Ramadan). But they were at times tolerated, at times oppressed, mainly by the rise and fall of Ottoman - Safavid relations, and how much of a threat they were. The further this 'Ali loving groups were from the Ottoman-Safavid frontier (for instance, some were in Rumelia, the Ottoman Balkans), the more leeway and tolerance they were given. To view this as a clear schism even in 16th and 17th centuries is probably a mistake.
These political and ideological lines became clearer over the centuries, but it's not as abrupt a rupture as, say, the schism between Protestantism and Catholicism. Maybe it's closer to the split between the Catholic and the Orthodox churches, where we have a clear date for when they "split" (the mutual excommunications of the Pope and the Patriarch of Constantinople in 1054) but really they had been drifting apart for centuries before that (already many things only affected one area but not the other--Rome, for instance, was never affected by the iconoclast controversies that were so important in Constantinople).
So in short, I would be surprised if crusaders made a big deal out of it because at this point it was mainly difference that related to the political and religious elite, rather than society at large. And it while it was a meaningful distinction, it certainly wasn't the only one, or even the primary one, in the Muslim world at the time. The most likely time it would have mattered to the Crusaders is when the Shi'a Fatimid Dynasty fell and was replaced, at least in the Levant (including the area the Crusaders sought to conquer), by the Sunni Ayyubids, of whom Saladin is the most leader, who were supported by the Sunni Abbasid Caliphate (the Fatimids chief rival in terms of religious legitimacy). But I'm not sure the Crusaders would have seen changes from the Fatimids to the Ayyubids/Abbasids in primarily religious, rather than dynastic, terms.
At the time of the Crusades, they probably knew about the differences and enmities.
During the siege of Antioch, a delegation was sent by the Fatimids to the crusaders to propose an alliance against the Seljukids.
Guillaume de Tyr writes (rough translation, I read it in French) "For very long, there had been a grave hostility between the Easterners and the Egyptians: this ingrained hatred stemmed from the difference of their superstitions and their contradictory dogmas."
Albert d'Aix writes about the meeting and indicates that Fatimids and Turks are foes but without theological precisions. Raymond d'Aguilers only mentions the embassy.
Sources :
Islam was essentially portrayed to the Crusaders as one monolithic bloc, at least in the buildup to the First Crusade in the 1090s. The political divisions within the Muslim world did happen to make the timing of the First Crusade very fortuitous - but this was almost certainly not intentional.
However, upon arriving in Asia Minor and the Levant, the Crusaders learned a great deal about these political divisions, both from the Byzantines and from the local Armenian Christians. The Fatimids, who were in control of Jerusalem at the time, had essentially no stake in the success of the Seljuq Turks, who were the first Muslim regime that the Crusaders encountered. As such, there was diplomatic contact between the Fatimids and the Crusaders. After the Crusaders took Nicaea and Antioch, in particular, the Fatimids still hoped to contain them within Asia Minor. At this point the Crusaders had mainly been retaking Byzantine territory that had been seized by the Seljuqs, and the Fatimids hoped that this would be sufficient for them.
The Crusaders did show willingness to make agreements with particular Muslim regimes when it suited them, so they were clearly able and willing to conceptualize Islam not only in religious terms but in political ones as well, but needless to say, as Jerusalem was the ultimate goal of the Crusade, no actual peace could be achieved between them and the Fatimids.
I'm not a historian, so I'm just summarizing what I've learned from The Crusades by Thomas Asbridge.
I think there's a fundamental problem with this question since Jews, Christians, and Muslims were living side by side and engaging in philosophical debates from the founding of Islam through the medieval period. Participants in this debate would be extremely well informed of highly minute differences. Christendom was not monolithic and the experiences of a Christian in Andalusia, Denmark, and Byzantium would have been drastically different. Jewish and Christian scholars were reading texts, either in Syriac (form of Aramaic) or Persian, Arabic or in even further translation into Latin or Hebrew. Scholars would have been reasonably well versed in major themes of Muslim philosophy and theology although this would be more familiar to them as academic debates.
Islam wasn't just fractured on theological lines, as different empires overthrew each other and Islam spread in different directions, philosophical and theological debates grew in different ways, Mu'tazilite thinking was challenged by the Ismā'īlīs, and further challenged by the Ash'arites providing you with much of the basis of the Sunni-Shia intellectual basis you had the philosophy of Avicenna in the East (and the challenges of more radicals like al-Ghazālī) and Averroes in the west shaping and refining philosophical and theological debates that took a long time to develop.
The persian princes would even occasionally try to reinstate Zoroastrianism, and that continued into at least the 10th Century.
By the time that the Sunni and Shia split was put in more formal political terms it was the 19th Century, and yes, the people who cared were very clear on the differences as European thought leaned heavily on the translation movements of the Islamic world and multifaith coexistence predating the period of radicalization after the first crusade but even then lasting into later periods.
Conversely, how much did the Muslim world know about the Protestant reformation and such?