Considering that it is rare for a motorists to be charged with drink-driving after having their blood alcohol level checked, it is odd that this is the test referred to for a change in the limit.
The usual test (the one administered by police at the roadside and, with more accurate equipment, in a police station) is of a motorist’s breath, where the limit is 35 micrograms per decilitre. Testing blood is an alternative, as is testing urine (currently 107mg per decilitre).
Reducing the blood-alcohol limit to 50mg (0.05 per cent BAC) would need to be accompanied by a corresponding reduction in the other limits: presumably (by rounding) to 22mcgs per decilitre of breath and 67mg of urine. However, I have seen no reports of this.