Just one way of doing it: if you apply the Color Balance (HLS) effect you would be able to keyframe/expression the saturation property.
Put the B/W effect on an adjustment layer and keyframe the opacity of that.
Or as Kib suggested, use an effect that has a keyframable saturation property. Colour Balance is one, Hue/Saturation another.
Thirdly, you could use Tint effect which by default turns the image b/w and then animate the tint property.
You don't need to do this on an adjustment layer. You can animate the opacity of each effect itself on the layer on which it's applied; just expand the Compositing Options property group in the effect in the Timeline panel, and animate the Effect Opacity property.
very nice, thanks, that worked like a charm
thanks to the rest as well