The signal for Strong Wind used to bear the alias 'black ball' (photo here). It started 1st Jan. 1950 as the 'local strong wind signal', by the Observatory.
It indicates that wind with a a speed of 22 to 33 knots, i.e. 40.744 to 61.116 km/h.
At first, the use of this warning was in the nature of an experiment, hoped to be of value to fishing folk and owners of small craft.
One early record of this Strong Wind signal hoisted was 13 Sept, 1952 at 6:45am.
The night signal consists of three lights, white, green, white arranged vertically at the main stations. (+)
At the supplementary stations the signal consists of two green lights arranged vertically.
Some stations which show night signal only are noted on this gwulo photo c.1957 (link). As the 1931 observatory report, night signals are exhibited on the
tower of the Kowloon Railway Station, and repeated on HMS Tamar and over the Harbour Office, apart from the Signal Hill.
It was renamed to Strong Monsoon Signal (Black Ball) on 15 Apr. 1956. Same name is still kept in use until today.
Typhoon signals including the 'black ball' is not physically hoisted now and 'black ball' was no longer mentioned by the HK broadcasters either, for over 20 years. (*)
This practice ended when the last signal station on southwest Cheung Chau (^) was closed from 1 Jan. 2002.
The historical background of the 'black ball' may be read from first news article on China Mail.
Please advise me for corrections if any mistaken info in above.
(+) this appears to coincide with the then typhoon No. 2 signal's, which was in use since 1931 (for SW strong wind)
(*) there is a short HKO movie about the physical typhoon signals and a signal lights panel before; although narrated in Cantonese, it do show the real objects once in use in the Cheung Chau Wind Station before (link)
(^) moved to this 2nd location in 1971; its 1st location was near the former site of the Cheung Chau Assembly Hall
sources
China Mail, 1950-4-4 & 1952-9-13
Administrative Report : Report of the Director of Royal Observatory for the Year 1931
Hong Kong Observatory website