No; there's no requirement that the participants be unaware of the items being tested, except during the actual blind testing, of course. Here, since the violinists were to pick the Strad, they obviously had to be informed of this.
An example of this relating to audio equipment is the Stereo Review amplifier blind listening tests which have been cited here several times. Not only were the listeners told of the amplifiers($220- $12,000), but listened to them openly before the blind sessions, of course reporting many areas of superiority in the more expensive units, which disappeared when the labels and price tags did. This significantly increased the value of the test.
