Although some historical writings suggest early Morris teams "Blacked Up" - until the no Cotswold or North West Morris teams did so (except Bacup Coconut Dancers....). That is still true today.
Border Morris (that collected on the Welsh Borders) was so named by E.C.Cawte in 1963, and before then there were only 13 Border Dances collected. In some cases it was called locally "Niggering".
In the early 1970's two teams led the trends - Silurian Morris and the joint teams led by John Kirkpatrick - The Shropshire Bedlams (Male) and Martha Rhoden's Tuppenny Dish (Female) - began dancing - Both the Men's teams Blacked Up.
This started a trend and there was an explosion all over the UK of Border Morris teams..mostly copying the costume of two male teams above...of Black Faces and Rag Jackets...Even though not all the earlier teams had this costume. It was claimed that is was essential to wear a Disguise to dance, hence the Black face...generally done by using Burnt Cork. Eventually, many teams started using Black (pancake) Make up.
This is how we got to where we are today. Although a large percentage of teams have moved away from Black..and are now using other colours and Masks (for example - the Shropshire Bedlams now wear Masks)
Opponents of Blacking Up suggest Now is the time to find a different form of Disguise.

Tim Radford (Sorry if this has been fully covered before on this thread)