A first degree AV block is when the electrical signal is simply slowed a bit more than usual but not to the point that you end up skipping any heart beats.

Zelster et al. reported that AV block that was truly caused by drugs was found in only 14 (15%) of 92 patients who had second- or third-degree AV block during therapy with β-blockers, verapamil ...

Nearly 90% had atrial fibrillation at baseline, 5% had first-degree AV block, 33% had right bundle branch block (RBBB), and 7% had LBBB, left anterior fascicular block (LAFB), or left posterior ...

A PR interval greater than 200ms is considered to be a first degree atrioventricular block (AV block), which means that the signals are delayed (though not blocked), as explained (here).