There was a fairly high-profile repeat of all four series on BBC2 in 1998-1999, which was especially notable as it included the rarely seen first series - many years later my brother was surprised to find out they were from the 1980s as he assumed it was a new series at the time, particularly as Back & Forth arrived the following year.
That was a fantastic time for the 9-10 year old me to discover some classic comedy, they also repeated The Young Ones and Bottom during that time along with a limited run of Python airings to commemorate the 30th anniversary. It was also around the time of the 8th series of Red Dwarf, which even though is commonly called the worst series it was good to get some new episodes during that time.
Yes, they did repeat the entire Blackadder series during 1998, starting with Christmas Carol getting its first repeat for a decade (so it was the first time I'd seen it) and then, as you say, the first repeat of series one for many years. Again, this was the first time I'd seen it, having already seen the other three, so I couldn't get into it at all and abandoned it after after two episodes. What was particularly interesting about that run is that, as it was 35 minutes long, all the programmes for the rest of the evening started at five and 35 minutes past the hour -
https://genome.ch.bbc.co.uk/schedules/bbctwo/england/1998-01-16#at-21.00
A friend of mine on Twitter suggested that was the only time the news has ever been delayed due to an overrunning comedy show, rather than the other way round.
There was another repeat run of all the Blackadder series in the summer of 1990, on Sunday nights on BBC1. Series 1-3 were all shown pre-watershed and, given it was shown during the World Cup, some episodes of Blackadder II, including Beer, went out at 7.15 which seems demented. The repeat of Series 1 was to coincide with it being released on video for the first time, and I have a Radio Times where someone complains that they'd just bought it and they could have just taped it off the telly.
The Python repeats you mention in 1999 were particularly interesting because it involved the Pythons picking their favourite episodes, and Terry Gilliam picked the last ever episode, which I believe hadn't been repeated on the Beeb since 1975 and was certainly the first time I'd seen anything from the final series. Terry Jones picked the Cycling Tour from series 3 which was also quite exciting to me as the previous Python repeat run in 1994-95, the first time I'd seen it, gave up in the middle of series 2 (as had the previous repeat run in 1989-91).
The Young Ones repeats in 1998 were a bit notorious because they were new edits of the show, cut down from 35 to 30 minutes. Despite being personally supervised by Paul Jackson I know a lot of fans were unhappy about them as they lose quite a few of the most famous and popular sequences from them.
1993 was a notorious summer for the Beeb because there'd been an accounting cock-up which meant they had to lose a big chunk out of the programme budget, and so there were loads of repeats that summer, even more than usual in summer in those days, and they got loads of complaints, but I thought it was a great summer because, at the age of fourteen and just getting really interested in alternative comedy, they repeated loads of it. I remember that summer it got to the stage where there were alternative comedy repeats every night across BBC1 and BBC2 - I remember it was KYTV, French and Saunders, Canned Carrott, A Bit of Fry and Laurie and Alas Smith and Jones, in that order. The Smith and Jones ones were particularly interesting as they went right back to series 1 in 1984, and like French and Saunders and Fry and Laurie, it was the first time I'd seen those.