In June 1763, warriors of six nations surrounded Fort Pitt (modern Pittsburgh, Pennsylvania) and demanded that the British evacuate. The Swiss commander of Fort Pitt, Captain Ecuyer, persuaded the Indians to return home, giving them trade goods, including blankets. A Captain Trent noted that two of the blankets and a kerchief were taken from the "Small Pox Hospital." (So many settlers had been driven into Fort Pitt by Indian attacks that the British were obliged to quarantine the sick outside the walls of Fort Pitt).

It was certainly the aim of the British high command to introduce smallpox into the Indians. The British general in New York, Sir Jeffrey Amherst, said "you will Do well to try to innoculate the Indians, by means of Blankets ... that can serve to Extirpate this Execrable Race". However it appears that smallpox was already raging inside the Indian territories on the Ohio River by that time. A prisoner of the Indians named Gershorm Hicks stated in a deposition that smallpox was among the Mingo tribe in the spring of 1763. And it appears that the Indians contracted smallpox in raids on English settlements on the frontier. A captive of the Delaware tribe named James McCullough stated that warriors had returned from "depredations along the Juniata" River (in central Pennsylvania) and had suffered from the pox after returning home.

Moreover, the chiefs (Turtle Heart and Maumaultee), to whom the 'infected' blankets were given, were alive a month later when they returned to resume the siege of Fort Pitt. Either they had acquired immunity to smallpox previously or the blankets and kerchiefs were not virulent. The chiefs and their tribesmen maintained the siege throughout July without the kinds of losses that would indicate infection by smallpox. And Indian attacks along the frontier only grew more intense as the summer of 1763 progressed, until Colonel Bouquet's forces smashed the Indians at the Battle of Bushy Run in early August, 1763.

(source: "Never Come To Peace Again: Pontiac's Uprising and the Fate of the British Empire in North America", David Dixon, Norman Oklahoma, 2005, pp 151-155)

edit: ecch well I see a mod has posted while I wrote this.

The best attested incident of a deliberate attempt to spread smallpox via blankets and other textiles occurred in 1763, during Pontiac's War, which I've discussed briefly here.