The evidence for the combination of heat treatment and pressure flaking at Blombos is difficult. One must measure flake scars---examples of tiny dings in the edge stone tool that were likely made by pressing. Mourre et al. base their interpretations on the best data on hand: experimental replication of the stone tools, and microscopic investigation of the edges on the tools recovered from the site.
The Mourre study looked at what are arguably projectile points recovered from the Still Bay occupations at Blombos. The points were clearly hafted, and there are impact scars on many of them that indicate that they were used as hunting weapons. They are by and large made of silcrete, a raw material that is local, but can have a sugary texture that is not necessarily conducive to stone-working.
Mourre and colleagues examined the tools under a microscope to identify flaking patterns, and then they conducted experimental archaeological studies, replicating the tools themselves by heat-treating and pressure flaking to recreate the shape and form of the working edges. While their interpretations must be hypothetical, their techniques are standard, totally acceptable methodologies used on later sites without dispute.
Pressure flaking at Blombos Cave is likely to hold up on further scrutiny. The suite of modern behaviors at Blombos---bone tools, ochre, shell beads, and other aspects---lead one to accept the presence of pressure flaking as further evidence of the burst of modern behaviors seen on the South African coast, 75,000 years ago.
Sources and Further Information
Mourre V, Villa P, and Henshilwood C. 2010. Early use of pressure flaking on lithic artifacts at Blombos Cave, South Africa. Science 330:659-662.