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.