Two years ago implanted a clematis which was blue and it flowered blue for two years

the pigments that make blue/purple are sometimes sensitive to the acidity/alkalinity of the soil and this may be the reason. some pigments are also affected by temperature too.
Some plants also change petal colour as the season progresses.

which clematis is it? do you know the varietal name?

can you show pictures of the old flowers and this years. though as I am blue/purple colour blind [a mutation!] I wont be able to tell. but some one else might be able too.

welcome to GoY too :o)

Sbg, are Clematis ever grafted? If so, it might be a sucker.
Nice garden, Animalsdad, but only one clematis flower is barely visible, and that shaded, hiding its exact color.

I've not heard of them being grafted but that doesn't mean they are not.

The picture of the blue clematis is the only one I've got, I will post a picture of its new dark colour next time I am home. It is definitely not a sucker. I may have its varietal name in my records,I will have a look. Thanks for the suggestions to date.

So that is the blue color. I thought so, but I have been fooled by pictures with mixed sun exposures before. Are there still any blue flowers mixed in with the purple ones?

No blue flowers only the purple ones and it is very prolific.

Well, according to my research, grafted clematis are common among the commercially produced ones available at the garden center. If yours was grafted, and something killed it to the ground last year, what grew back might have been entirely the rootstock--that is, the variety that it was grafted onto.

I appreciate what you say but the plant was in perfect condition and I pruned it as normal at the end of February and as far as I can tell there was only one plant on the rootstock.

Were the flowers somewhat sparse last year?

No it had many flowers the one in the picture was one of the last.

Well, every one of the ideas that I had are getting shot down. :)
The only thing that I can think of now is to do a close inspection of the base of the vine. The thought is that after pruning this February, the main part of the plant died--from who knows what--and it regenerated itself from shoots from underground. If that is true, the thickish main stem will be dead, or not growing well enough to bloom, and it will be surrounded at the base by one to several vigorous new stems, which will be making up the bulk of the vine.
At least, that's what I have seen many times in roses and citrus where I live. Often, the owner is completely unaware of what is happening while it happens, and only notices when the plant starts bearing different flowers or fruits.

I will have a look when I get a chance. Thanks for all the ideas. I will just have to get to like the new colour but I did prefer the blue one.