On Roman re-enactment forums I frequent, this Hollywood 'black praetorian' armour is referred to derisively as 'Darth Vader Praetorian'. It is an invention of the props designers of
Gladiator and has no basis in history at all. There is some debate over the standard colour of Roman soldiers' tunics (including those of Praetorian Guards) and general consensus is that they were white, russet red or both (red for field use and white for parade). But never purple or black.
Hollywood also has this weird idea that armour has to be dull or dirty all the time or that
segmenta was made of leather - again, see
Gladiator. Armour was, on the contrary, kept as shiny as possible and was polished before battle, because that was a way of emphasising it to often unarmoured opponents.
Blued armour was fashionable in the late medieval period, largely because fashions come and go. Blued or 'black' seems more prevalent in some medieval manuscript illuminations because armour was often depicted using silver leaf, which looked great at the time but has tarnished to black over the centuries. There is absolutely no Roman armour that I know of that was blacked or blued in this way.
For detailed information on how Praetorians actually looked see Boris Rankov,
The Praetorian Guard