A dictionary of Confusable Phrases: More Than 10,000 Idioms and Collocations

blue devil • blue devils
blue devil —
1
. (also: blueweed) a coarse prickly weed with
spikes of blue flowers: Blue devil is a coarse biennial Old World
herb introduced as a weed in eastern North America. 2. (Drug
users) a blue capsule or tablet containing barbiturate (also:
bluebird): I am spending far less than I was while taking those
little blue devil pills.
Note: 
The expression does not correlate in meaning with the
phrase blue devil fish— a reef fish (Paraplesiops bleekeri) inhab-
iting exposed reefs from 3 to 30 meters deep: Blue devil fish are
easily identified by the blue and white-banded body with yel-
low pectoral fins.
blue devils —
1
. a feeling of depression; despondency: By the
time that he had eaten his soup and drank a glass of wine, he
had got rid of the fit of blue devils. 2. the apparitions seen in
delirium tremens: They, by a continued series of intoxication,
become subject to what is popularly called the Blue Devils.
blue heavens • blue skies
blue heavens —(Drug users) blue capsules or tablets contain-
ing barbiturate: Amobarbital tablets and capsules are called
“blue birds,” “blue devils,” or “blue heavens.”
blue skies —(also: blue sky) 
1
. said of impractical or unfeasible
plans and ideas: The research amounted to a “blue skies” proj-
ect because it involved going beyond the present capacity of
the reactor design. 2. (U.S. stock market) is used of fake or
worthless bonds and stocks: They were what I would term
“blue sky and hot air” securities.
blue laws • blue-sky laws
blue laws —(U.S.) laws characterized by extreme strictness
in matters of morals: Even the religious fanatics of Boston and
the mad zealots of Hertford christened them the “Blue Laws.”
blue-sky laws —(U.S.) laws designed to prevent fraud in the
sale of stocks: The “Blue Sky” law of Kansas prohibits the sale
of stock or bonds of any company chartered outside the State.