I'm currently watching this episode and searched for further explanation.
My thoughts are that many of the customs of "worship" if you will, of the indigenous people of Mexico where commingled with Catholicism as the country was conquered by Spain.
At the time of Cortez, Catholicism was a welcome and natural fit for these people after undergoing centuries of human and blood sacrifice. Blood made their world go round, if you will.
For Catholism it's all about the Eucharist, or "Body" of Christ. Part of the Eucharist as well is the "Blood" of Christ. Mexico's indigenous people, at the time, embraced this conversion favorably, since it contained a common theme of "Sacrifice". And certainly a much preferred way of worship.....
Many indigenous customs, however, were blended with Catholicism. Part of why Mexico has such a violent and fascinating history. Skulls still represent death in Mexico.