[mch62 (Mark)]

I won't get to technical but in short a Apocromatic is better corrected for chromatic (colour ) aberrations than an Acromatic lens.

Usually but not necessarily an Achromatic lens is 2 elements.
A three or more element is called Apocromatic in the case of the barlows your chasing.
Although as to weather it has enough colour correction to be called Apocromatic is another story.
There are also semi Apocromatic and then there are Ed lens (extra low dispersion) but what there all trying to do is focus all the spectrum to one point , i't's how well they do this as to weather it could be called Apocromatic.

There are also 4 element barlows called Powermates and technically there not a barlow but do a similar but better job than the previous but at a cost.