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Ultralight bosons can form large clouds around stellar-mass black holes viathe superradiance instability. Through processes such as annihilation, thesebosons can source continuous gravitational wave signals with frequencies withinthe range of LIGO and Virgo. If boson annihilation occurs, then the Galacticblack hole population will give rise to many gravitational signals; we refer tothis as the ensemble signal. We characterize the ensemble signal as observed bythe gravitational-wave detectors; this is important because the ensemble signalcarries the primary signature that a continuous wave signal has a bosonannihilation origin. We explore how a broad set of black hole populationparameters affects the resulting spin-0 boson annihilation signal and considerits detectability by recent searches for continuous gravitational waves. Apopulation of $10^8$ black holes with masses up to $30\mathrm{M}_\odot$ and aflat dimensionless initial spin distribution between zero and unity produces upto a thousand signals loud enough to be in principle detected by thesesearches. For a more moderately spinning population the number of signals dropsby about an order of magnitude, still yielding up to a hundred detectablesignals for some boson masses. A non-detection of annihilation signals atfrequencies between 100 and 1200 Hz disfavors the existence of scalar bosonswith rest energies between $2\times10^{-13}$ and $2.5\times10^{-12}$ eV.Finally we show that, depending on the black hole population parameters, caremust be taken in assuming that the continuous wave upper limits from searchesfor isolated signals are still valid for signals that are part of a denseensemble: Between 200 and 300 Hz, we urge caution when interpreting a nullresult for bosons between 4 and $6\times10^{-13}$ eV.

Characterizing the continuous gravitational-wave signal from boson clouds around Galactic isolated black holes