filter coefficients for biquad

Hi all,
I'm studying the coefficients for a basic recursive LOW PASS filter (see Dodge's book p. 210) and I'm using a0 and b1 coefficients of biquad object.

I made some experiment using an impulse as input signal, in order to verify the impulse response of the filter.

I notice in the LOW PASS filter the b1 coefficients is always a negative number, this means the first delay of the impulse should be negative (out of phase)... but the impulse response is always positive!

Does someone knows why the impulse response is always positive, although the b1 coefficient is negative?

Thanks in advance.
Federico