In PhD, you can choose for the discipline you have interest for. So if you want to work in Hematology then obviously you can. As you are completing your MSc MLT then you are eligible to qualify the Biology paper of CSIR NET & on qualifying the examination followed by an Interview, you can take admission in your desired institute.

For more information regarding CSIR NET you can consider the link below:

https://competition.careers360.com/articles/csir-ugc-net

I hope this helps. Wish you a great eve!