The Mormon pioneers arrived with three slaves when they entered the Salt Lake Valley in July 1847. They were Hark Lay, Green Flake, and Oscar Crosby. While slavery was never widespread in Utah Territory, it was legal and Brigham Young, the LDS Church president, had no objection to slavery. You can read my previous answer on Young and the emergence of the LDS priesthood and temple ban here.

The Wikipedia article you linked to is correct: it's impossible to know how many slaves were in the territory between 1850 and when Congress outlawed slavery in the territories in 1863, but it was probably several dozen. These slaves came from southern converts to the LDS Church who had emigrated to Utah from the South before the Civil War. Missionaries had converted Green Flake's owners, for example, and then that family decided to gather with other Mormons in Nauvoo, Illinois.

These slaves usually were given the same last name as the family that owned them; Green Flake was given to James Flake as a wedding present. When Hark Lay's owner, John Crosby died, he was given to Crosby's daughter and her husband, William Lay. Oscar Crosby was also a slave of John Crosby, and he became the property of John Brown and Elizabeth Crosby Brown. Hark Lay later took the last name Wales, but it's not clear why. He was forcibly separated from his wife when several Mormons were called to found the colony of San Bernardino, California, in 1851 under Charles C. Rich and Amasa Lyman. Several of the southern converts had traveled to Utah with Rich and Lyman in 1847, and now they would travel to San Bernardino with them, bringing their slaves. As many as a quarter or a third of slaves in Utah went to San Bernardino.

Slavery in California was outlawed, so technically those slaves should have been freed when they entered the state, but they were not, and most seem not to have known their status. When the colony at San Bernardino was abandoned in 1857, about half of the Mormons stayed behind in defiance of Brigham Young, and they remained with their slaves. in a few years, those slaves were freed and several went on to live as free people in California. Others, like Hark Lay Wales, returned to Utah, where they were later freed and continued to work and live.

Records show that Green Flake was given to the LDS Church as a tithing donation. It's unclear exactly what this means; was ownership of Flake actually transferred to the church, or was he lent out for his labor? There are other records that suggest slaves were given to the LDS Church as tithing, though they are maddeningly vague. One unnamed "African servant girl" was donated by John Brown for $1,000. Green Flake, whether he was loaned out or actually owned by the church, worked for Brigham Young, Heber C. Kimball, and likely other church leaders. He too gained his freedom, though it's unclear if he was freed prior to 1862 or not.

You'll notice I'm using phrases like "it's unclear," "seems," "may have," and so on. Tracking the lives of individual slaves through the historical record is difficult. Many did not know how to read or write, and in most cases were forbidden from learning how to read or write. Letters, diaries, journals for slaves in Utah are nonexistent. Their marriages were not legally recognized, and so there are almost no records to document them. They were not allowed to own land or property, so no records exist to document them through land sales or claims, etc. Instead, mentions of slaves crop up in the records of white people: a mention in a diary here, a passing mention in a letter there. One letter about Hark Lay to Brigham Young shows that Young counseled, at least in this instance, against breaking up slave families. But we need to be cautious about extrapolating too much from one letter.

You'll also notice that I'm talking exclusively about Flake, Wales, and Crosby. Perhaps because these three men were the first slaves in the Salt Lake Valley (some records suggest they were even sent ahead of the main body of Pioneers, and they entered on July 22, 1847, instead of the more common date of July 24, 1847) they have far more information about them than any other slaves. Green Flake in particular has the most documented existence, and a photo of him even exists. But large gaps exist in documenting his life, and tracking other slaves is difficult.

One exception is Biddy Mason. Mason came to Utah with her owners, then went on to San Bernardino in 1851. When word came that the colony might be shuttered, Mason's owner, Robert Smith, said that they would move to Texas. Mason fled, hid, and then sued for her freedom. She won in court (an unlikely outcome that almost certainly would not have happened if Smith had not failed to appear at the hearing) and she moved to Los Angeles and became a prominent citizen. Mason never returned to Utah and it's unknown if she had been baptized into the LDS Church.

Some slaves, like Green Flake, were baptized. This is another challenging angle: some slaves became members of the church, but it's impossible to know how voluntary this was. On the one hand, slaves could only be baptized with their master's permission, but they also may have been forced to join the church by their owners, and it seems unlikely that missionaries would object to such forced membership.

Understanding the day-to-day lives of these slaves is also difficult. There are no records of beatings or whippings of slaves in Utah, but it doesn't mean that it didn't happen. Some letters mention slaves being "saucy" or difficult. At least one slave ran away and was later sold, it seems directly as a result. Brigham Young seems not to have been odious to slaves, but there is a spectrum of attitudes in between full endorsement of chattel slavery and full endorsement of abolition. Young was a racist who believed that Blacks were cursed and dirty.

There were no large plantation-style farms that employed dozens or hundreds of slaves in Utah. Instead, Families owned no more than a few slaves, who usually did farm or household labor. The word "servant" is often used to describe these slaves, including the Brigham Young monument in downtown Salt Lake City that lists Flake, Wales, and Crosby as part of the Pioneer Vanguard company. We can assume that the slave experience in Utah was varied; Green Flake later said that his life in Utah was a good one. Other slaves were not so complimentary.

One reminiscence, published in the Black-owned paper The Broad Ax, said that slaves would gather together to "discuss their condition, and gaze in wonderment at the lofty mountains, which reared their snowy peaks heavenward, and completely forbade them from ascertaining how they could make their escape back to the South, or to more congenial climes. For we were assured that their lives in the then new wilderness, was far from being happy, and many of them were subjected to the same treatment that was accorded the plantation negroes of the South."

It's important to take these reminiscences seriously, especially over the word of white owners who often painted a rosy picture of slave life (for obvious, selfish reasons). Understanding the lives of slaves in the United States is crucial. It is easy for Americans today to picture slaves as sort of zombies, forced to go through life under the lash, with no personality and no identity beyond that of victims of slave owners, until they were freed by the white saviors of Lincoln and the Union soldiers fighting the Civil War. In fact, slaves carved out remarkably vibrant existences for themselves in spite of their condition. They had culture, families, music, religion, friendships, etc. They were instrumental in fighting for their own freedom.

Finally, there were also free Blacks in Utah. Initially they were all members of the LDS Church. The most well-known was the James family since Jane Manning James had been a servant in Joseph Smith's home in Nauvoo. But as the transcontinental railroad was built and mining expanded in Utah, other free Blacks came to the state. They participated in politics, they started families, and they owned land. Utah, however, was far from a utopia. Like the rest of the nation, it could be dangerous for Black men and women. In 1866, a Black man named Thomas Coleman was found on Capitol Hill in Salt Lake City, murdered and with a sign pinned to him that said, "Notice to Niggers leave white women alone."

I'm at the max word limit, see my comment below for reading.