Withholding your own speech (tweets) from a certain audience (the blocked user/s), is well within your own 1st Amendment right.
Your right to free speech includes the right to choose whom to speak to - so long as no laws are being violated (you can't for example, refuse to testify if subpoenaed).
Additionally, Trump's Twitter account is not protected. If you are being blocked by him, you can literally sign out of Twitter and view them.
Imagine the outrage if it was Muslims.
Private website. 1st Amendment doesn't apply.