Hey zanaffar81, thanks for checking out the game.
In answer to your questions, the spell 'stat block' determines the effects of the spell.
- Portal spell, you have interpreted this correctly. The term 'resisted' was added to help clarify that a level 3 or above Monster can resist (i.e. being cast into a portal against their will).
- Blackfire spell, the stat block should over-rule the description here. I could tighten the wording to 'engulfs all targets within range' and leave the spell stat block to inform how many targets and range.
- Please note that all spells can be 'amplified' (Page 51 of the Players Handbook) and so targets and range for example can be increased by spending more health.
Hope this clarifies your questions and thanks for the feedback.