I am wiring a hot tub I recently purchased and is a 220V 4 wire connection. The wire is AWG 6. The instruction book shows black and red hot wires for this tub, but the wiring on this tub has black and blue hot wires. The reading I have done always indicates black and red hot wires. I'm guessing some wire companies make blue hot wires. Is there any significance to this not being red that I may not be aware of?
Ungrounded "hot" conductors can be identified by any means that is not used to identify grounded "neutral", or grounding conductors. So basically if it's not white, gray, any color other than green with three continuous white or gray stripes, green, green with yellow stripes, bare, or any of the other identification methods below, it can be an ungrounded "hot" conductor.
Blue is an acceptable color for the insulation on an ungrounded "hot" conductor.
National Electrical Code 2014
Chapter 2 Wiring and Protection
Article 200 Use and Identification of Grounded Conductors
200.6 Means of Identifying Grounded Conductors.
(A) Sizes 6 AWG or Smaller. An insulated grounded conductor of 6 AWG or smaller shall be identified by one of the following means:
(1) A continuous white outer finish.
(2) A continuous gray outer finish.
(3) Three continuous white or gray stripes along the conductor's entire length on other than green insulation.
(4) Wires that have their outer covering finished to show a white or gray color but have colored tracer threads in the braid identifying the source of manufacture shall be considered as meeting the provisions of this section.
(5) The grounded conductor of a mineral-insulated, metal-sheathed cable shall be identified at the time of installation by distinctive marking at its terminations.
(6) A single-conductor, sunlight-resistant, outdoor-rated cable used as a grounded conductor in photovoltaic power systems as permitted by 690.31 shall be identified at the time of installation by distinctive white marking at all terminations.
(7) Fixture wire shall comply with the requirements for grounded conductor identification as specified in 402.8.
(8) For aerial cable, the identification shall be as above, or by means of a ridge located on the exterior of the cable so as to identify it.
Article 250 Grounding and Bonding
250.119 Identification of Equipment Grounding Conductors. Unless required elsewhere in this Code, equipment grounding conductors shall be permitted to be bare, covered, or insulated. Individually covered or insulated equipment grounding conductors shall have a continuous outer finish that is either green or green with one or more yellow stripes except as permitted in this section. Conductors with insulation or individual covering that is green, green with one or more yellow stripes, or otherwise identified as permitted by this section shall not be used for ungrounded or grounded circuit conductors.
Here is a breakdown of common U.S. household electrical wire color coding: http://www.doityourself.com/stry/220-wiring-color-code-interpretation
It is not always common to see blue, yellow, and other colors outside of your common black, red, white, green, and bare copper; however, there is nothing wrong.
The blue wire you see is likely the second hot wire and will be connected to the red from the house. If you are unsure, and seeing as I don't have the manual and/or pictures, I suggest calling the manufacturer and asking. I am also assuming the additional 2 wires on the hot tub are a white neutral and a green or bare ground.
The NEC only mentions a few colors. White and Grey may only be used as a grounded conductor (neutral wire) and green or green with a yellow stripe may only be used as a grounding conductor. Typically in a house application its industry standard to use Black and Red for the two line conductors but in a commercial application, specifically in 3 phase there would be three hots...black, red, blue. The manufacturers operate under different sets of codes than field installers so they pretty much just pick a color.