 He was imprisoned in 1578 for his work as a priest.After being released from prison, he lived and worked from the house of Mrs. Meany in Westminster, England in secret.In 1601 a government spy described Blackwell as "about 50 years of age, his head brownish, his beard more black, cut after the fashion of a spade, of stature indifferent, and somewhat thick, decently attired"

After the death of Cardinal Allen in 1594, the leadership of the clandestine Catholic Mission in England was thrown into disarray.