My great toe nail is opacified and bluish-black, and oozes clear red fluid following trauma (my bike fell on it two weeks ago). Is this serious?

Trauma to the great toenails can be from a direct blow
such as you described, or from jamming of the nail against the toe of footwear during
sports.  Such trauma often causes bleeding beneath the nail (subungual hematoma).  This
is what has happened to you.  The blood accumulating beneath the nail has obscured the
pink nail bed, which is no longer visible through the translucent nail.  The nail
therefore looks dark bluish-purple.  The fluid oozing from under the nail is a mixture
of serum and degenerated blood.

Fortunately, the condition
is not serious.  The hematoma will eventually resolve.  The worst possible result would
be that the nail has separated enough that it will eventually fall off.  A new nail will
form beneath the separated nail, and eventually an entire new nail will be formed.  This
will take a few months.  The new nail may not be entirely smooth and uniform.  So there
may be a somewhat disappointing cosmetic result.