My understanding is that a believing and yet unbaptised person who is martyred for the faith of Christ was determined to have been baptised by blood. I believe Mathew Ayairga is an example of a person who might be said to have been baptised in blood. He was in the group of Coptic Christians who were beheaded on the beach in Libya those he himself was not, and yet in the face of the significant courage and faith of the twenty Coptic Christians when confronted declared 'their God is my God'. It was more common in earlier periods when people often chose to defer baptism for fear of post baptismal sin, the plan being to live robustly and be baptised at the last minute. The blood of the martyrs is the seed of the church. We continue to pray that Mathew Ayairga and the other 20 may Rest in Peace and Rise in Glory!
Another martyr whom the ACSA declared to be baptised by blood: https://en.wikipedia.org/wiki/Manche_Masemola
The 20th Century ran red with the blood of the Martyrs and on early showing the 21st Century may surpass it. Thank God for the witness of the martyrs. May their witness remind us all to be strong in the faith of Christ. Thank you for bringing it to our attention Aidan.
Most recently the little old French priest sacrificed while celebrating Mass. May he remain at peace in the worlds of God, Requiescat in Pacem