The black underwing was used for Fighter Command single seat fighters up until April 1941.
The first two Tomahawks for No.403 (RCAF) Squadron arrived 13th March 1941, ex No.234 Sqn RAF. Next two after that arrived on 19th, one more on the 20th, bent their first one in a taxiing accident on the 21st, one more arrived on the 25th and two more on the 31st. Most were coming 'pre-used' from other Squadrons, primarily ACC Squadrons. They didn't go officially operational on the Tomahawks until 11 May 1941. A lot of standing patrols and practice interceptions when aircraft serviceability permitted. Interesting visitor recorded on 21 May 1941, with attachment of Lt Zemke of USAAC to Squadron as an 'observor' for US Government. Participation in a number of local training exercises including an aerodrome defence exercise against 'enemy paratroopers'. Spitfires started to be received 25 May 1941, with flying on them ramping up rapidly and Tomahawks being ferried away pretty quickly.
Black underwings re-appeared at various times on the Tomahawks operated by the RAF ACC Squadrons flying them after the April 1941 time point, usually associated with participation in large scale exercises where the black underwing was used to signify which 'side' the Tomahawk was participating in the exercise - friendly or enemy (usually 'enemy' marking). Some of the Tomahawks still flying with the RAF ACC Squadrons in early 1943 for Exercise SPARTAN, by which time most of them had re-equipped with Mustangs, but still had a couple of Tomahawks on strength fitted with a vertical camera installation - had the underside of one wing painted black if they were one of the 'enemy' Squadrons. By that stage they had been repainted into the current Day Fighter Scheme C&M including the later style markings.