Why does a blue sky at dusk appear nearly black through a telescope?

Unless you have a filter on the lens, the color shouldn't change noticeably. I think your explanation is the correct one: the telescope simply doesn't transmit anywhere close to 100% of the light it's receiving. Note that with perfect optics, the sky would be brighter by approximately the ratio of the areas of the objective to that of the eye-piece. Because the moon is basically the only astronomical object this type of telescope is really effective for, I wouldn't be surprised if it was designed to dim the source---as the moon can be very bright, and could easily hurt the user's eye. (When observing the moon with larger telescopes, one generally uses a darkening 'moon-filter', for just this purpose).

In telescopic (binocular) observation brightness of observed extended surface (like sky, Moon's surface, etc.) strongly depends from applied magnification. Lower magnification makes observed surface brighter. Higher magnification makes surface darker.

Applying 150x on your my cheap toy telescope you must get much more dark image of sky surface than observed with naked eye.

How much darker - depends from size (diameter) of telescope exit pupil. Diameter of exit pupil can be calculated from telescope aperture (diameter of it lens or main mirror) divided on applied magnification.

E.g. in case scope with aperture 100 mm and magnification 50x we have diameter of exit pupil equal 100/50 = 2 mm. Compare to size of observer eye pupil (6-8 mm at night conditions). We see that observation through this telescope reduce useful area of observer pupil in 3 times (9 times in square!) and surface brightness shall be 9 times less then observing with naked eye.