My 3.5 year old spitz had been spayed recently. On 5th day of the surgery she got some fresh vaginal bleeding. But soon the bleeding reduced into blackish bloody discharge. My vet is out of the state and by phone he prescribed tranexaemic acid and enrofloxacin. Today is 7th day of surgery and she is having a very little bloody discharge till now. I am little worried as I am not getting much information about this in web. Can help me with some advice

The blackish discharge will probably simply be old, clotted blood from the first vaginal bleed; the fact that there is some fresh red blood again is slightly concerning. I assume you've been restricting her exercise as advised; no running, jumping on and off furniture, no going up and downstairs and so on, and lead walks only? At least she'll be back at the vet for her 10-day check very soon, but if you have any worries before then, take her back for a check sooner.

As said, she'll be needing to go in for the stitches to be removed in another few days so unless the discharge is copious and she's really lethargic, it can probably wait.   However, if the vet removed all her repro.organs, there should be nothing left to bleed, other than the incision, internally?   I've only have post whelping/Section bleeding like this, for about a week as again with a 'Section, it should all have been removed when the surgery was performed.

The original bleeding wouldn't worry me quite as much as it starting up again with a bright colour.  If you are really concerned, get back to your vet.   You've paid for the surgery after all.