I have seen Blaze solo thrice, but also two times with Maiden, in 1995 and 1998.
What was it like? Well, back in 1995, it was only the second Maiden concert I visited. I had seen the band one time before that, and that was in 1992. I didn't see the band in the eighties, so I didn't have a huge "Bruce concert past" as many older people around that time. I was 20, and I was just terribly excited to see the band, and -in a time without internet- I was mighty curious about the songs. So the whole thing was more about the gig and the music, than about Blaze. To be honest, I'd seen Blaze on MTV (Ray Cokes!) doing Man On The Edge and Wrathchild and that went very good. Blaze had a different voice, maybe even a different approach than the other singers but as long as it sounded good, it was OK for me. The Di'Anno material certainly fitted his voice well. Also his own songs went pretty good. Someone earlier in this topic said that the TXF songs sounded so much better live than the studio versions. This was true, but believe me: this is the case with most Maiden songs.
Blaze came across as a humble and motivated person, with as much fire in the eyes as Bruce.
It showed he was in a band he loved and he respected the audience very much. I liked his performance, but the 1980s Bruce-era songs were hard to do for him. Too high for his range. Why do I say 1980s Bruce-era, and not just Bruce-era? You'll see that later*. During the concert it didn't show that much (I was at least as focused on the other guys, and very in awe to see Maiden again), but listening to bootlegs it's easier to focus on it.
From the regular TXF-tour setlist the following songs went not that good or even very bad:
Heaven Can Wait
The Evil That Men Do
2 Minutes to Midnight
The Clairvoyant
The Number of the Beast
Hallowed Be Thy Name
These songs went from pretty OK to very good:
Man On The Edge
Wrathchild
Lord of the Flies
Fortunes of War
Blood on the World's Hands
Afraid to Shoot Strangers
The Aftermath
Sign of the Cross
The Edge of Darkness
Fear of the Dark
Iron Maiden
Running Free
From the non-Blaze era songs I particularly remember that he was very good in Wrathchild, Fear of the Dark* and also Afraid to Shoot Strangers*. From the TXF songs, I thought he did Man on the Edge and Blood on the World's Hands the best.
Three years later, Maiden was back. This time I stood closer to the stage and had a very good view on Blaze and the other moving band members. Blaze looked much more comfortable in his role as frontman and the band was (as always) on fire. I remember a particular awesome part in the set where Sign of the Cross, Afraid to Shoot Strangers and Hallowed Be Thy Name were played in a row!
This time I'll focus on the songs which went not good, or (very) bad:
Lightning Strikes Twice (he couldn't do the chorus)
Heaven Can Wait
2 Minutes To Midnight
Hallowed Be Thy Name
The Evil That Men Do
Of course, in other songs he had some edgy moments, but such things are only notable on bootlegs. Again I was in awe of seeing the band, and I swear, that even though I realized that Blaze didn't do some songs that well, I was still happy with the performance and I didn't mind he was in the band. At the time I liked both Blaze albums, but now I am more critical with VXI.
Judgement:
Was Blaze a bad performer?
No
Was Blaze a bad singer?
No
Was Blaze a bad singer when it comes to 1980s Bruce era songs and other high parts in some other songs?
Yes
The setlist didn't do much good. Even though these difficult songs were less than the half of the set, these were the moments many people remember. Especially the people who only compared Blaze with Bruce and who didn't want to accept Blaze as a new singer, joining Maiden in a new chapter of the book.
Other aspects (taken from Blaze's official biography) Blaze had to deal with: failing techniques (soundwise), short rehearsing time, and he had to deal with an annoying roady, I think it was a monitor man.
Blaze himself did his utter best but he had to work in difficult circumstances and he had to deal with an enormous legacy. I really wish people would read his biography. It gives honest insight into his world at the time.
Despite all the criticism, right before and after he was replaced, the first time I saw Blaze solo, after his debut, I was immediately convinced that he was a good performer, and singer.