  This paper describes an analysis performed at 250 GeV centre of mass energy
for the reaction e+e- -> bbbar with the International Linear Collider, ILC,
assuming an integrated luminosity of 500 fb-1. This measurement requires
determining the b quark charge, which can be optimally performed using the
precise micro-vertex detector of the detector ILD and the charged kaon
identification provided by the dE/dx information of its TPC. Given that the
forward backward asymmetry is maximal for e-L (Left-handed electron
polarisation), it has been necessary to develop a new method to correct for
unavoidable angular migration due to b charge mis-measurements. This correction
is based on the reconstructed events themselves without introducing external
corrections which would induce large uncertainties. With polarized beams, one
can separate the Z and photon vector and axial couplings to b quarks. The
precision reached is at the few per mill level, and should allow to
confirm/discard the deviation observed at LEP1 on the ZbRbR coupling. Model
independent upper bounds on the tensor couplings, F2V and F2A, are derived
using the shape of the angular distribution.
