  High-level first-principles computations predict blue phosphorene bilayer to
be a two-dimensional metal. This structure has not been considered before and
was identified by employing a block-diagram scheme that yields the complete set
of five high-symmetry stacking configurations of buckled honeycomb layers, and
allows their unambiguous classification. We show that all of these stacking
configurations are stable or at least metastable configurations both for blue
phosphorene and gray arsenene bilayers. For blue phosphorene, the most stable
stacking configuration has not yet been reported, and surprisingly it is
metallic, while all other arrangements are indirect band gap semiconductors. As
it is impossible to interchange the stacking configurations by translations,
all of them should be experimentally accessible via the transfer of monolayers.
The metallic character of blue phosphorene bilayer is caused by its short
interlayer distance of 3.01 {\AA} and offers the exceptional possibility to
design single elemental all-phosphorus transistors.
