If black is
an acceptable color for you (or even if it isn't !) I'd suggest leaving that one alone -- as an example of original condition, etc -- and painting the blue top black, to match. No, the two won't be identical in every way, but they'll make a nice and usable (and salable) pair -- and you'll have one which was untouched.
Good ? The legs appear identically aged, so -- either leave them alone, or paint both. I'd leave them as is -- for the foreseeable future.

I agree. Repaint/restore the ...
I agree. Repaint/restore the blue one, leave the black as-is. I did a similar thing with a set of aluminum group chairs. One shows fifty years of use, the other looks like it was just delivered.

If the underside
of the blue seat is also blue, you might leave that surface uncoated, so as to preserve the original color as a sample.
There seem to be more kinds of "blue" than of other colors -- for some reason . . .

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Original paint
is only ORIGINAL once.
Personally, I like to see age and wear on a vintage piece. Especially if it is made of a material that will change over time such as your painted metal, or unpainted wood. Rust, dents, veneer loss, bad scratches, water damage and the like are another story, but naturally occurring wear from use and age I do not find unsightly in the least.

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