Calculating 30% less solar energy. Provided the math is right, would there have been liquid water on Earth?
Quick and simple answer: A greater greenhouse effect.

There is an equilibrium in place. CO2 weathering rates depend on temperature, so at lower temperatures, we see CO2 levels rise, warming the planet. At higher temperatures, higher weathering rates mean more CO 2 drawdown, so the planet cools.

This is a very long term process. But this CO2 thermometer effect is believed to be the reason why we have had liquid oceans for all of known earth history.

As far as the black body effect goes; this is correct - from the viewpoint of an observer at distance, Earth would 'appear' colder. However, this is because the emission height changes (see https://andthentheresphysics.wordpress.com/2014/03/05/effective-emission-height/ ) for more of an explanation. Think of it as 'the height at which the atmosphere becomes IR-opaque' (although this is a simplification); the temperature then follows the lapse rate down to the ground. So raising the emission height raises the surface temperature. 

(NB late at night, add refs later..)