Would you have sex with someone you knew had HIV on the condition that a condom be used? Would you knowingly handle blankets infected with a deadly pathogen, just so you could go and give them to people you wanted dead? I think not.
Knowing, no. But as I was just discussing with another person, in those days they had no idea what caused disease, and they sure didn’t know the blankets had anything in them to cause smallpox. The more I think about it the more I think they probably didn’t know if the Indians suffered from smallpox before the British got there!