I know that it is allowed in SEO to use comma as separator but, is it ok to add comma as a separator between keywords in
title without leaving a space?
<title>High blood pressure,Hypertension</title>
You may use any sign as separator, but you should always keep the readability in view. Suffering readability will for sure suffer your SERP CTR - and this would be too bad, to get an impression but to loose the click.
Google will recognize your keywords anyway - it will do it based on words placed befor and after words, which are weirdly separated, like in your example. Compare SERPs on these three searches:
They are the same - ergo Google recognizes them all. But seeing such in the page title would not necessarily motivate to clik it.
In Google, you use three things to separate data in the search engine.