I was thinking of getting 2x6"x8' redwood planks to make raised beds. The beds will not be burried at all. Will lining the inside that with that black plastic make these beds last longer, or does it not matter?
Black plastic is a wide ranging term. Landscape fabric will not help you here as it is porous. Redwood is pretty expensive so putting a liner or inner pot will extend the life of the containers. A properly lined container will last much longer than one where the wood is exposed to damp soil.
Pool liner comes in a variety of types. The blue coloured material used for swimming pools is toxic for fish. So, probably not a good choice for plants. I have used black EPDM pool liner marked for use in ponds to line containers and it works great.
- UV stable for 25+ years
- resistant to leaks and tears
- not that expensive given the life of the material
- impervious to water