Section 6 of the Fugitive Slave Act of 1850 specifically barred the alleged fugitive from testifying in his own defense: "In no trial or hearing under this act shall the testimony of such alleged fugitive be admitted in evidence."
In his wonderful (so far) Fugitive Justice: Runaways, Rescuers, and Slavery on Trial, lawprof Steven Lubet points out that this "provision was not quite as draconian as it now must seem" given the rules of evidence at the time. Nowadays, parties to suits are permitted to testify, with the trial judge giving the jury a instruction directing them to consider the party's interest in the case when evaluating that testimony.
In 1850, however, the general rule was that interested parties were barred from testifying altogether, on the theory that their testimony was inherently unreliable. The no-testimony provision of the Act thus "merely put an alleged slave (the subject of the proceeding, but not a formal party) on the same footing as the alleged master."
Virtually every aspect of the Fugitive Slave Act [of 1850] tilted against the alleged runaway, who was denied the right to a jury trial, to appeal, or to seek relief from another court. The law specifically provided that "in no trial or hearing under this act shall the testimony of such alleged fugitive be admitted in evidence." The latter provision was not quite as draconian as it now must seem. In 1850, plaintiffs and defendants in lawsuits, including criminal defendants, were not permitted to testify on their own behalf in any state or federal court, pursuant to the so-called interested party rule. Strictly speaking, then, the no-testimony provision of the Fugitive Slave Act merely put an alleged slave (the subject of the proceeding, but not a formal party) on the same footing as the alleged master (the plaintiff or claimant), neither being allowed to testify.
Two caveats are in order. First, as Prof. Lubet notes, the no-testimony provision of the Act nonetheless left the alleged fugitive at a terrible disadvantage:
But of course their true positions were far from equivalent. The claimants had resources to call upon other witnesses - agents, employees, neighbors - to testify to the identity and servile status of a prisoner, while the fugitives almost invariably had only themselves.
Second, Prof. Lubet may be incorrect that in 1850 parties were barred from testifying "in any state or federal court." Although it may have remained the general rule, it appears that at about this time some states were beginning to reconsider the old common-law rule and a few had already modified it. In A History of American Law, Prof. Lawrence M. Friedman cites both Connecticut and New York as having abandoned the rule by 1850:
A few of the more restrictive rules of evidence were, to be sure, relaxed. At one time, as we noted, nobody who was an interested party was allowed to testify at trial. England abolished this rule in 1843, Michigan in 1846, and other states followed over the next thirty years. The rule that disqualified the parties to a lawsuit was only a special case of the rule; it too was abolished. Connecticut did this in 1848; and the Field Code, in New York, in the same year, did the same. Other states gradually joined the trend, Illinois, for example, in 1867.
About the illustration, entitled Practical Illustration of the Fugitive Slave Law (published in Boston, 1851):
A satire on the antagonism between Northern abolitionists on the one hand, and Secretary of State Daniel Webster and other supporters of enforcement of the Fugitive Slave Act of 1850. Here abolitionist William Lloyd Garrison (left) holds a slave woman in one arm and points a pistol toward a burly slave catcher mounted on the back of Daniel Webster. The slave catcher, wielding a noose and manacles, is expensively dressed, and may represent the federal marshals or commissioners authorized by the act (and paid) to apprehend and return fugitive slaves to their owners. Behind Garrison a black man also aims a pistol toward the group on the right, while another seizes a cowering slaveholder by the hair and is about to whip him saying, "It's my turn now Old Slave Driver." Garrison: "Don't be alarmed Susanna, you're safe enough." Slave catcher: "Don't back out Webster, if you do we're ruind." Webster, holding "Constitution": "This, though Constitutional, is "extremely disagreeable." "Man holding volumes "Law & Gospel": "We will give these fellows a touch of South Carolina."Man with quill and ledger: "I goes in for Law & Order." A fallen slaveholder: "This is all "your" fault Webster." In the background is a Temple of Liberty flying two flags, one reading "A day, an hour, of virtuous Liberty, is worth an age of Servitude" and the other, "All men are born free & equal." The print may (as Weitenkampf suggests) be the work of New York artist Edward Williams Clay. The signature, the expressive animation of the figures, and especially the political viewpoint are, however, uncharacteristic of Clay. (Compare for instance that artist's "What's Sauce for the Goose," no. 1851-5.) It is more likely that the print was produced in Boston, a center of bitter opposition to the Fugitive Slave Act in 1850 and 1851.