A hybrid is simply a cross between two pure races, a mongrel is where at least one parent is cross bred. In many instances I think the word "hybrid" is misused, probably because "mongrel" is seen as derogatory - it probably helps sales as well!
All breeders know about hybrid vigour, but unless the mating of queens is done under controlled conditions a true hybrid is difficult to achieve in bees.
For many years the American bee press carried advertisements for Dadant "Starline" and "Midnite" queens. The Dadant & Sons bee supply company employed Dr Gladstone H (Bud) Cale Jr and it was him who is credited with developing these hybrids that were produced by complicated crossings. The Starline was a very light Italian type that was said to be a hybrid between two Italian lines. They were very prolific and suited the American commercial beekeepers who were working bees 12 months of the year. In American conditions they were very productive. The Midnite was a grey bee whose parentage is somewhat obscure. Some sources state it was a hybrid between two Caucasian lines, others between Caucasian and Carniolan. These were originally bred for the U.S. amateur market, but improvements in productivity made them attractive to their commercial beekeepers. For some time in the U.K. after the hard winter of 1962/3 both were imported from the U.S in large numbers, either as queens or package bees. Some time later the York Bee Company took over sales, but I don't know the details.
Both the Starline and Midnite were quite useless in the U.K. The Starline converted every bit of food into brood, both summer and winter and the Midnite seemed to produced more propolis than honey. A recent search suggests they are no longer produced and haven't been for some time. This was apparently due to the high costs of producing these queens and the arrival of acarine and varroa into the U.S.
The Buckfast was said to be a hybrid, but bearing in mind the number of places Br Adam visited to collect material, it seems it was effectively a mongrel. I have handled Buckfast that have varied considerably in colour.