One of my bitches is due in season, she's right on 6 months since the last one -she has gone between 5 and 7 months in the past. She is due to be mated this time, it will be her first litter. Today she has had a slight discharge which is very dark brown, almost black -a bit like very old blood. It's not a lot. I have only ever seen anything similar a few years ago when a bitch who had been mismated and had the Alizin injections came into season the next time -the vet said it was old retained blood being cleared out (this was a fair amount!) and she was put onto antibiotics, was mated and had a healthy litter. This bitch however has never been mated. Should I take her to the vet to play safe or has anyone else had a season start with old blood appearing?

Mine often start with brownish discharge gradually becoming redder, but as you have a million times more experience than me, you'll know if it's unusual for your bitch. :-)

Mine often start with brownish discharge gradually becoming redder

Mine too at the very beginning of their seasons.  I never thought about this being 'old blood' to be honest.  But I agree, if this is unusual for your bitch and especially if you plan on breeding her, I'd get her checked.   If she has infection going on, it would be 'kind' to the owner of the stud dog (if an outside dog) not to allow him to mate with her, risking him picking up something?

Could be the tissue from the womb lining - don't know but think it likely.

It's gone today, absolutely no trace at all.

We sometime start with the clearing out the pipes I think

The good news is, today an absolutely normal looking season.

Think it may have been the womb tissue you had not noticed before, usually only last a short while and is I believe just dead cells that come away prior to the season, sure all will be well, good luck with the mating.