"Blackguard" Telecasters are those produced between 1950 and 1954, named after their black pickguard. Fender began installing white pickguards on Telecasters in 1956, so any tele before that is considered a "blackguard" tele.
Blackguards had flatpole alnico 3 pickups and ash bodies, which provided a somewhat different sound from that of a modern tele. Also, at least until 1952, their pickup selector was wired differently from the modern 3-way configuration. The switch still had 3 positions, but the first two were both neck-pickup-only selections, one voiced "dark" and one normal. The third position was the bridge pickup, with the neck pickup blended in depending on the position of the tone control.
Blackguards are the most sought after tele's for collectors purposes, and are also quite pricey if you can find one. Fender does make a reissue of the 1952 model that is quite accurate, including the pickup wiring schematics, ash body, and neck shape (blackguards had really deep necks, U-shaped.)
Next time someone talks about a blackguard tele, make sure they know what they are talking about. Some use this expression to refer to any tele with a black pickguard. Now you can correct them (or not.)